Identifying future closers isn't easy. Mariano Rivera, baseball's all-time saves leader and only unanimous Hall of Famer, began his pro career as a starter. So did Edwin Diaz and Wade Davis, the league leaders in saves a year ago, and many other pitchers who are nailing down the ninth inning for big league clubs this season.

But we're a fearless bunch here at MLB Pipeline. We present the prospect who's the best bet to become a future closer for each of the 30 clubs below, and we're not limiting our selection to pitchers currently in Minor League bullpens. Astros right-hander J.B. Bukauskas is the lone Top 100 Prospect among our candidates.

American League East

Jackson McClelland, RHP, Blue Jays (Double-A New Hampshire)
Big and physical, the 6-foot-5, 220-pounder works with an explosive fastball that can reach triple digits and a sharp breaking ball. He misses more barrels than bats right now, though that could quickly change as McClelland learns to better harness his power stuff and throw more strikes. He’s held hitters to a sub-.220 average and just seven home runs in roughly 140 innings since being selected in the 15th round of the 2015 Draft.

Zach Pop, RHP, Orioles' No. 20 (Double-A Bowie)
One of the five prospects acquired from the Dodgers in last summer’s Manny Machado trade, Pop stands out for his plus-plus sinker that he throws in the mid-90s from a lower arm slot. It enables him to induce grounders at an elite rate, as his 65.8 percent ground-ball rate in 2018 was the sixth-best mark among Minor League hurlers (60 IP min.). He pairs his heater with a power slider, and together they make him lethal on right-handed hitters.

Colin Poche, LHP, Rays' No. 17 (Triple-A Durham)
Poche was perhaps the best Minor League reliever in 2018, when he recorded a 0.82 ERA, 0.79 WHIP and .151 BAA in 66 innings (40 appearances) between Double-A and Triple-A. Along the way, he racked up 110 strikeouts (against 19 walks), fanned 45.6 percent of his batters faced and averaged 15.0 strikeouts-per-nine. At the root of the left-hander’s success is a fastball that sits at 92-94 mph but plays at a higher effective velocity thanks to a high spin rate and because Poche has deceptive arm actions and gets considerable extension over his front side.

Durbin Feltman, RHP, Red Sox No. 12 (Double-A Portland)
The best reliever in the 2018 Draft, Feltman is dealing in Double-A at the beginning of his first full pro season. With a running fastball that can reach 99 mph and a deceptive mid-80s slider, he could be closing games in Boston in the near future.

Luis Gil, RHP, Yankees' No. 13 (Class A Charleston)
Acquired from the Twins in March 2018 in exchange for Jake Cave, Gil can hit 101 mph with his four-seam fastball and generates crazy spin rates on his power curveball. He has difficulty keeping his delivery in sync and commanding his pitches, so he may eventually move from the rotation to the bullpen.

American League Central

Lenny Torres, RHP, Indians' No. 12 (Rookie AZL Indians Blue)
Torres has exceptional arm speed that generates 92-98 mph fastballs with riding action and the makings of a plus slider with power and high spin rates. He has the athleticism to repeat his delivery, though he's a bit small for a starter at a listed 6-foot-1 and 190 pounds.

Richard Lovelady, LHP, Royals' No. 16 (MLB)
Lovelady has pitched his way to the big leagues -- he made his debut on April 9 -- largely on the nastiness of his fastball-slider combination. He gets a lot of swings and misses on his heater, which touches 97 mph, largely because of his funky delivery coming from a lower three-quarters arm slot that jumps at the hitter. His mid-80s slider has improved and also misses bats.

Bryan Garcia, RHP, Tigers' No. 21 (Triple-A Toledo injured list)
A fifth-round pick in 2016 after he had recorded 43 career saves as University of Miami’s closer, Garcia pitched at four levels and reached Triple-A in his first full season, only to have his 2018 campaign wiped out by Tommy John surgery the next spring. When healthy, the 23-year-old righty features a fastball-slider combo with natural bat-missing ability.

Jorge Alcala, RHP, Twins' No. 18 (Double-A Pensacola)
Acquired from the Astros last July in the Ryan Pressly deal, Alcala has the potential to stick as a starter and has already had success in the role in Double-A. He has a four-pitch mix headlined by a 95-98 mph fastball as well as a track record of missing bats, but control problems combined with the inconsistency of the 23-year-old’s secondary pitches prevent him from working deep into games, leading many to project him in a bullpen role long-term.

Zack Burdi, RHP, White Sox No. 16 (Triple-A Charlotte injured list)
The White Sox made Burdi the 26th overall pick in 2016 with the notion that he'd quickly become their closer, and he reached Triple-A at the end of his first pro summer before blowing out his elbow early the next season. He hasn't made it all the way back from Tommy John surgery, but before he got hurt he ran his fastball up to 102 mph and his slider into the low 90s.

American League West

Ty Buttrey, RHP, Angels' No. 14 (MLB)
Acquired from Boston in last summer's Ian Kinsler deal, Buttrey made his big league debut two weeks after the trade and quickly established himself as a major bullpen asset by logging four saves across 16 appearances down the stretch. He’s been even better thus far in 2019, striking out 12 in 7 1/3 innings across nine scoreless outings. The 26-year-old righty’s fastball can flirt with triple digits and effectively sets up a changeup that he throws with splitter action.

J.B. Bukauskas, RHP, Astros' No. 6, MLB No. 93 (Double-A Corpus Christi)
Bukauskas has one of the best sliders in the Minors, a mid-90s fastball that peaks at 98 mph and can flash a plus changeup with fade. His staunchest proponents see him as a Sonny Gray-esque starter, while others point to his size (6 feet, 196 pounds) and effortful delivery and think he's a definite reliever.

Grant Holmes, RHP, Athletics' No. 12 (Double-A Midland)
Originally a first-round pick in 2014 by the Dodgers -- who dealt him to the A’s in the August 2016 deal for Josh Reddick and Rich Hill -- Holmes was making a quick rise through the Minors before a right rotator cuff injury wiped out nearly all of his 2018 campaign. Though he’s currently healthy, concerns about another shoulder injury may ultimately force the 23-year-old to the bullpen, where his heavy 93-94 mph heater and plus breaking ball could play up.

Joey Gerber, RHP, Mariners' No. 20 (Class A Advanced Modesto)
Seattle’s eighth-round pick last June out of the University of Illinois, Gerber recorded eight saves and averaged 15.1 K/9 last summer, finishing his pro debut in full-season ball. In terms of stuff, the right-hander pitches at 95-97 mph with premium spin rate and backs up his fastball with a slider that flashes plus. Assigned to Class A Advanced Modesto for his first full season, he could become one of the first players from his Draft class to reach the Majors.

C.D. Pelham, LHP, Rangers' No. 15 (Triple-A Nashville)
The Rangers have a deep collection of dominant relief prospects, starting with Pelham, who has the size (6-foot-6, 235 pounds) and athleticism of an NFL tight end. His mid-90s fastball reaches 99 mph with unpredictable life, and his low-90s cutter can be just as devastating.

National League East

Chad Sobotka, RHP, Braves' No. 21 (MLB)
Sobotka came from a small school (South Carolina-Upstate) and had a stress fracture in his back when the Braves selected him in 2014's fourth round, but he since has blossomed into a reliever with a fastball that can reach 100 mph with elite spin rates. To get trusted with the ninth inning, he'll need to demonstrate more consistency with his slider and command.

Jorge Guzman, RHP, Marlins' No. 9 (Double-A Jacksonville)
One of the hardest-throwing starters in the Minors, Guzman has a fastball that has clocked as high as 103 mph and also misses bats with a power slider. His command isn't nearly as impressive as his velocity, however, so scouts believe he'll wind up as a reliever sooner rather than later.

Ryley Gilliam, RHP, Mets' No. 21 (Class A Advanced St. Lucie)
The Mets may have a future bullpen force in Gilliam, the team’s fifth-round pick from last year’s Draft. Though slightly undersized, the Clemson product possesses two power offerings: a mid-90s heater that has late life and a downer curveball in the upper 70s. He’s yet to allow a run so far this season, racking up 12 strikeouts and giving up three hits over his first 8 1/3 frames in the Florida State League.

James Bourque, RHP, Nationals' No. 17 (Double-A Harrisburg)
The Nationals added Bourque to their 40-man roster last November after a breakout campaign in which he posted a 1.70 ERA with 76 strikeouts in 53 innings. He was particularly good after a July promotion to Double-A, registering a 0.92 ERA over 15 appearances for the Senators. He will show two plus pitches in any given outing: a 97-98 mph heater that he throws with late life, and a breaking ball in the upper 80s with sharp, downer action.

Enyel de los Santos, RHP, Phillies' No. 6 (Triple-A Lehigh Valley)
Originally signed by the Mariners and acquired from the Padres in a trade for Freddy Galvis in December 2017, de los Santos has mid-rotation upside with a mid-90s fastball and a quality changeup. His ability to hit 98 mph in shorter stints also makes him an intriguing option for the late innings.

National League Central

Braden Webb, RHP, Brewers' No. 12 (Double-A Biloxi)
The 2016 third-rounder reached Double-A for the first time late last season, putting a bow on a breakout campaign in which he racked up 128 strikeouts in 120 2/3 innings as a starter across two levels. He nets his whiffs with a plus fastball-curveball pairing, sitting in the mid-90s with the ability to snap off a powerful 12-to-6 curve. But with a high-effort delivery and corresponding control problems, Webb may be best suited for a late-inning bullpen role.

Griffin Roberts, RHP, Cardinals' No. 12 (Restricted list)
Roberts' high-spin-rate slider is a 70-grade pitch and was regarded by many scouts as the best college breaking ball in the 2018 Draft class. His fastball will sit in the low-to-mid 90s with late life and he has feel for throwing a changeup, too. While the Cardinals view him as a starter and will develop him accordingly, Roberts stands to make a more immediate impact in a bullpen role and could jump on the fast track once he returns from the 50-game suspension he received in December after his second positive test for a drug of abuse.

Riley Thompson, RHP, Cubs' No. 27 (Class A South Bend)
One of three Louisville pitchers (along with the Burdi brothers) on this list, Thompson is a starter for now but had trouble staying healthy and throwing strikes in college. He can reach triple digits with his fastball and spin some well above-average curveballs when he's at his best.

Nick Burdi, RHP, Pirates' No. 24 (MLB)
Like his younger brother Zack (see White Sox), Burdi is a former Louisville closer who went in the top two rounds of the Draft and underwent Tommy John surgery (in 2017). Drafted by the Twins and acquired from the Phillies in a Rule 5 Draft-related deal for international bonus pool money two Decembers ago, he pairs an upper-90s fastball with a power slider.

Joel Kuhnel, RHP, Reds' No. 26 (Double-A Chattanooga)
Purely a reliever since he entered pro ball as an 11th-round pick in 2016, Kuhnel is all about power. His fastball ranges from 95-101 mph, while his slider and changeup reside in the low 90s -- and he delivers all three offerings for strikes on a consistent basis.

National League West

Yoan Lopez, RHP, D-backs' No. 14 (MLB)
Signed in 2015 for more than $8 million after pitching professionally for three years in Cuba, Lopez took off with a move from the rotation to the bullpen last year and quickly reached the Majors and opened 2019 in Arizona’s ‘pen. Pairing an upper-90s heater with a wipeout, two-plane slider at 84-87 mph, he can both pile up whiffs and consistently miss barrels.

Dennis Santana, RHP, Dodgers' No. 6 (Triple-A Oklahoma City)
Signed out of the Dominican Republic as a shortstop in 2013, Santana moved to the mound after one pro season and has made rapid progress ever since. He could be a mid-rotation starter, though his crossfire delivery, hard-to-control life on his 93-98 mph sinker and inconsistent (though sometimes nasty) slider may point him to the bullpen.

Melvin Adon, RHP, Giants' No. 15 (Double-A Richmond)
Adon showcased one of the more electric fastballs in the Arizona Fall League last offseason, ranging from 95-102 mph with a four-seamer and unveiling a heavy two-seamer as well. He also showed more consistency with his upper-80s slider and control in shorter stints, leading San Francisco to make him a full-time reliever this year.

Andres Munoz, RHP, Padres' No. 24 (Double-A Amarillo)
One of the hardest throwers in the Minors, the 20-year-old right-hander sits in the upper 90s heat and consistently reaches triple digits with a fastball that has explosive late life. His hard slider in the upper 80s gives Munoz a second dynamic offering that has room for more improvement. He still has gains to make as a strike-thrower, but the pure stuff alone could soon have him in the big leagues.

Justin Lawrence, RHP, Rockies' No. 13 (Triple-A Albuquerque)
A self-taught sidearmer, Lawrence gets so much movement on his upper-90s sinker that it sometimes gets mistaken for a slider. He has a real slider as well but relies heavily on his sinker, which could make him a closer if he resists the temptation to overthrow it and lose life and command.