For some teams, the last month of the season serves as preparation for the postseason. For others, it the last steps of a season that will end short of October glory. But no matter the trajectory a team is on, every lineup in baseball has at least one player who has underperformed, whether it's due to injury, ineffectiveness, or some other reason. Here's a look at a player on each MLB roster who has something to prove during the final weeks of the 2023 season.

All stats as of Wednesday unless otherwise noted.

AL EAST

Blue Jays: 1B Vladimir Guerrero Jr.

Where did the 2021 version of Vladdy go? Guerrero has fallen short of expectations again this season, but all will be forgotten if he finally puts the Blue Jays on his back. Just look at what Julio Rodríguez has done in Seattle, shaking off a poor start to the season by winning AL Player of the Month in August. Guerrero is long overdue for a similar run. No player on this roster holds more power to change this team’s trajectory than Guerrero. -- Keegan Matheson

Orioles: RHP Jack Flaherty

The Orioles thought they fortified their rotation when they acquired Flaherty from the Cardinals before the Trade Deadline. He pitched well in his first game against the Blue Jays on Aug. 3, allowing a run in six innings. Unfortunately, it’s the only game as an Oriole he has gone that deep into a start. Since that game against Toronto, Flaherty has allowed 17 runs in 28 1/3 innings. He even missed a turn in the rotation because of general soreness. Flaherty relies on his four-seam fastball and slider the most, but both pitches have been hit this year, with opposing batters hitting a combined 107-for-353 (.303) with 11 home runs on those two pitches. –Bill Ladson

Rays: OF Randy Arozarena

Anybody who has seen Arozarena play in previous postseasons, this spring’s World Baseball Classic or this summer’s All-Star Game understands that Arozarena can turn it on when the lights shine brightest. Nobody would be surprised to see Arozarena kick it into another gear come October and carry the entire lineup, whether the Rays get there as the AL East champions or a Wild Card team. But it certainly wouldn’t hurt Tampa Bay to see one of its top hitters start peaking before then. He hit .288/.401/.484 the first three months of the season, and even with a better August, he’s batting just .217/.322/.351 since July 1. -- Adam Berry

Red Sox: LHP Chris Sale

In May, it seemed Sale had recaptured his groove, going 4-0 with a 2.25 ERA over six starts. But in his first start of June, the lefty suffered a stress reaction in his left shoulder blade. Though Sale rehabbed aggressively from the injury so he could help the Red Sox down the stretch, his performance has been uneven since his return and his velocity isn’t what it was earlier in the season. Sale hopes to build off the five scoreless innings he threw in his last start in Kansas City so he can go into the offseason with some confidence. Sale is under contract for one more season, and he’d be the first to admit the Red Sox have yet to get much return on their investment on that five-year, $145 million contract. He has roughly five starts left this season. -- Ian Browne

Yankees: C Austin Wells

The hype is attached to Jasson Domínguez, but Wells arguably has a more demanding challenge ahead: not only swinging the bat, but handling a big league pitching staff. Thus far, Wells has received rave reviews from Bombers hurlers regarding his preparation and pitch-calling; Michael King said that Wells is “phenomenal at adjusting,” calling him “a great communicator.” Manager Aaron Boone applauded Wells’ “thirst” for in-game minutia, high praise as the 24-year-old will likely come into 2024 vying to share catching duties with Jose Trevino. -- Bryan Hoch

AL CENTRAL

Guardians: SS Gabriel Arias

The answer here could be any of Cleveland’s middle infielders, but Arias, especially, needs to prove there’s more life in his bat than he showed throughout the majority of the season. The team knows just how impressive his defense is and how critical it could be at shortstop for the foreseeable future, but the Guardians’ main focus needs to be improving the offense moving forward. If Arias can’t deliver offensively, the team has guys like Tyler Freeman, Brayan Rocchio and eventually Juan Brito to strongly consider at short. -- Mandy Bell

Royals: 1B Nick Pratto

Pratto has missed 36 games since he landed on the 10-day IL with a left groin strain, giving him 299 plate appearances this season after his callup at the end of April. With Vinnie Pasquantino returning to first base in 2024 and others in the outfield, like Nelson Velazquez, getting more looks as the season comes to a close, it’s important for Pratto to get healthy and show what he can do in this last month. The 2017 first-round pick still has plenty of upside, but the Royals need to see that in the Majors to confirm Pratto can be part of the future. -- Anne Rogers

Tigers: 3B Matt Vierling

Vierling was a nice, versatile player and offensive catalyst for the season’s first half and has filled a huge void at third base this summer, but his second-half struggles have raised the question whether he’s a regular to build around or a bench piece. With prospect Justyn-Henry Malloy returning to third base down the stretch at Triple-A Toledo, the Tigers are clearly looking at their long-term options at the hot corner, both in and out of the organization. -- Jason Beck

Twins: DH Byron Buxton

Well, it’s now or never for Buxton to prove that he can be healthy for the stretch run. The Twins still have hopes that he could step foot in center field in what remains of the season, but that hit a snag when he had to exit his second rehab game for Triple-A St. Paul with patellar soreness in his troublesome right knee. The full-time DH experience did not prove fruitful for Buxton’s production, and the Twins’ offense has been rolling of late; can Buxton show that he can -- and should -- be a factor when the games matter most? -- Do-Hyoung Park

White Sox: RHP Michael Kopech

The 27-year-old right-hander put together a solid first half despite walking 49 in 86 innings, striking out 97 and posting a 4.08 ERA through 17 starts. But Kopech’s fortunes have dipped through 10 second-half starts, with a 7.49 ERA, 39 hits and 11 home runs allowed and 40 walks issued against 32 strikeouts in 39 2/3 innings. Kopech and Dylan Cease are the only starters presently penciled into the 2024 rotation. So, the White Sox would like to see a strong finishing kick from a hard-working hurler who has not completed at least six innings since June 4 and has worked less than five in five straight starts. -- Scott Merkin

AL WEST

Angels: 1B Nolan Schanuel

Schanuel was surprisingly called up just 40 days after he was taken with the No. 11 overall pick in the Draft. He’s held his own so far, displaying excellent plate discipline. But he hasn’t hit for any power and it could be a concern going forward. But if Schanuel plays well down the stretch, he could put himself in a good spot to be the club’s first baseman next season. -- Rhett Bollinger

Astros: RHP Cristian Javier

Javier was 7-1 with a 2.84 ERA and 0.99 WHIP in his first 12 starts, during which Houston was 10-2. He allowed nine homers in 69 2/3 innings. He’s 2-2 with a 6.52 ERA and 1.52 WHIP in 14 starts since, allowing 13 homers in 67 2/3 innings (Astros are 8-6). If he can return his peak form, it changes the dynamics of a Houston rotation anchored by Justin Verlander and Framber Valdez. -- Brian McTaggart

Athletics: SS Nick Allen

Allen has flourished on the defensive side with an elite glove, but the offense remains a work in progress. Entering Wednesday, the shortstop was batting .212 on the year, though he has been showing signs of improvement over the past month, hitting .272 (22-for-81) with three home runs over his last 27 games dating back to Aug. 6. With A’s No. 11 prospect Darell Hernaiz enjoying a strong season at Triple-A, Allen’s performance down the stretch could determine his status on the depth chart entering 2024. -- Martín Gallegos

Mariners: OF Jarred Kelenic

The 23-year-old has proven in his third full season that he belongs and can be an above-average big leaguer, finally living up to the lofty prospect pedigree he carried a few years back. But after a self-inflicted injury that has sidelined him for seven weeks, there will be some added pressure on Kelenic to prove that he’s learned from the temper-induced experience and produce for a team with postseason aspirations. -- Daniel Kramer

Rangers: OF Adolis García

García notched his second All-Star appearance this year after a strong first half, but has struggled with a .197/.293/.462 slash line since Aug. 1. The Texas offense has coincidentally paralleled his struggles. Though García leads the AL in RBIs, down the stretch he’s struggled to come up with the big hit. If the Rangers want to remain in the postseason picture, it’s likely they’ll need García -- who exited Wednesday's game with right knee discomfort after crashing into the wall -- to return to form to do so. -- Kennedi Landry

NL EAST

Braves: RHP Kyle Wright

Wright has pitched effectively in two of his first three Minor League rehab starts and could join Atlanta’s pitching staff during the final weeks of the regular season. He’s been sidelined since early May with right shoulder inflammation. But he could get a few starts down the stretch, or at least be available to come out of the bullpen during the postseason. Adding last year’s only 20-game winner for the playoffs seems like a significant option for the team that will likely enter the postseason as the World Series favorites. -- Mark Bowman

Marlins: RHP David Robertson

Miami added the high-leverage righty to a southpaw-heavy bullpen at the Deadline, but the results haven't been there. After posting a 2.05 ERA in 40 outings for the Mets, Robertson entered Tuesday with a 7.50 ERA and three blown saves in 12 appearances for the Marlins. A return to form for Robertson, who recently lost the closer's role, is vital because of his experience pitching in the World Series, Olympics and World Baseball Classic. -- Christina De Nicola

Mets: LHP David Peterson and RHP Tylor Megill

Unfairly or not, Peterson and Megill have been linked all season given their status as rotation depth pieces who may or may not be a part of the franchise’s longer-term future. Both have largely struggled in the Majors. Both have also shown flashes of improvement of late. If Peterson, Megill or both can give the Mets a strong September showing, they will enter the offseason on much firmer footing for a team that figures to be aggressive on the starting pitching market. -- Anthony DiComo

Nationals: 3B Carter Kieboom

Kieboom, 26, returned to the Nationals in late August after missing the entire 2022 season because of Tommy John surgery. He is getting the opportunity for playing time at third base, while also looking to improve on his .207 batting average his last time in the bigs (he hit three homers in his first six games back). Kieboom, a 2016 first-round pick by the Nats, has appeared in 458 Major League games since debuting in ‘19. -- Jessica Camerato

Phillies: RHP Aaron Nola

The Phillies’ postseason hopes hung in the balance on Oct. 3, 2022, when Nola threw 6 2/3 perfect innings in Houston to clinch the third NL Wild Card. Nola then pitched gems in the NL Wild Card Series and NL Division Series to help the Phillies win the NL pennant. But Nola has been inconsistent at best this season. He is 12-9 with a 4.55 ERA, allowing a career-high 30 home runs with a career-high FIP (4.23). They need that early October 2022 version of Nola down the stretch. -- Todd Zolecki

NL CENTRAL

Brewers: SS Willy Adames

The Brewers’ club MVP each of the past two seasons hasn’t had the year he wanted in 2023. Adames missed time in May after being struck in the head by a foul ball while leaning on the dugout rail, and saw his OPS slip as low as .645 just before the All-Star break. He has turned it on of late, with five home runs and a .901 OPS since Aug. 18, and will look to continue that surge through the end of the season. Here’s why it’s so important for both Adames and the team that he does: He’s heading into his final season of club control in 2024. If last winter’s market was a guide, productive shortstops are in incredibly high demand. -- Adam McCalvy

Cardinals: OF Tyler O’Neill

The first thing that O’Neill needs to prove is that he can stay healthy for the remainder of this season. The musclebound slugger missed 2 1/2 months with a back injury earlier in the season and he’s been bothered of late with a troublesome left knee, limiting him to just 65 games. The Cardinals want O’Neill to be their everyday left fielder because of the Gold Glove defense he can bring and his rare combination of speed and power. O’Neill has been swinging the bat well of late, hitting home runs on Saturday and Tuesday. -- John Denton

Cubs: RHP Jameson Taillon

Taillon was the Cubs’ top pitching target over the offseason, and for good reason. His recent track record as a strike thrower and innings eater made Chicago’s four-year, $68 million contract reasonable. As the Cubs keep pushing for a postseason spot, Taillon has endured a trying, inconsistent campaign. The big righty had a 7.86 ERA in his last five turns, giving him a 5.73 ERA overall on the season. Across early July and into August, though, Taillon went 5-0 with a 2.17 ERA, looking like the arm the Cubs believed they were signing. That is the starter the Cubs need again with Marcus Stroman sidelined and a possible October rotation to map out. -- Jordan Bastian

Pirates: RHP Luis L. Ortiz

Ortiz took Pittsburgh by storm last September, consistently touching triple digits with his fastball and teasing top-of-the-rotation stuff. So far, the 24-year-old hasn’t been able to turn that potential into production, posting a 4.90 ERA across 68 innings and spending half the year with Triple-A Indianapolis. Ortiz has pitched better in his last two outings (three earned runs, 11 innings), and if he can finish the season strong, he can help solidify a spot in next year’s rotation. -- Justice delos Santos

Reds: SS Elly De La Cruz

The 21-year-old De La Cruz was baseball’s most electric player in the first month of his big league career but has largely struggled throughout the second half and down the stretch as a sub-.200 hitter. With injuries plaguing the Reds lineup throughout the past few weeks as they try to fight for an NL Wild Card spot, Cincinnati could use De La Cruz’s production and exciting style of play to help lift them down the stretch. -- Mark Sheldon

NL WEST

D-backs: RHP Zac Gallen

Gallen finished fifth in the NL Cy Young voting last year and really should have finished higher. Once again, he is putting together a season that is worthy of Cy Young consideration. The difference is this year Gallen has the opportunity to pitch in a Wild Card race, which means more eyes will be on him. If he continues to pitch well, the added exposure should result in more Cy Young support. -- Steve Gilbert

Dodgers: LHP Clayton Kershaw

Proving something might not be the right way to describe it because Kershaw will be giving a speech in Cooperstown one day, but the left-hander does have some questions to answer at this stage in his career. In his last two starts, Kershaw’s velocity has been down over 2 mph and Dodgers manager Dave Roberts has alluded to him not being 100 percent since coming back from a left shoulder injury last month. Given where the Dodgers are in their starting rotation, they’re going to need the 35-year-old to be the ace in October. Whether he’s still able to do that remains to be seen. -- Juan Toribio

Giants: OF Mitch Haniger

Haniger was the Giants’ biggest free-agent addition after signing a three-year, $43.5 million deal over the offseason, but he’s missed a combined 85 games with a left oblique strain and a fractured right forearm and hasn’t hit much when healthy. The Giants’ offense has been the worst in the Majors in the second half, so they desperately need Haniger and other veteran hitters to get going to avoid falling behind the pack in the crowded NL Wild Card race. -- Maria Guardado

Padres: LHP Blake Snell

Snell is a pending free agent, making a very serious run at the NL Cy Young Award and the Major League ERA title. He’s reiterated that his focus is solely on doing his part to pitch the Padres to the postseason. (And to be clear, Snell has done that.) But amid a disappointing season, San Diego’s playoff odds are practically nonexistent now. Still, Snell can take home some major personal accolades this year, which could set him up for a big-time payday this winter. -- AJ Cassavell

Rockies: OF Kris Bryant

At his core, Bryant is an easygoing fellow who normally doesn't say much on the field or off. But he has embraced a mentorship role with this young club by speaking up in hitters meetings and helping shorten the learning curve of standout rookie outfielder Nolan Jones. But the Rockies didn't sign him for seven years and $182 million for just that. Bryant intends to return from his left index finger fracture next week. Because of a series of injuries, Bryant has played in just 107 games since signing with the club prior to the 2022 season. The final three weeks will not answer the overarching question – whether Bryant can stay healthy. But if he performs anywhere near what the Rockies expected, Bryant can give the club a look at how a lineup full of young players can perform when he is a part of it. -- Thomas Harding