In 2016, Luis Severino battled injury, spent time in the bullpen and in the Minors, and posted a 5.83 ERA for the Yankees. Robbie Ray spent all season in the D-backs' rotation but produced a 4.90 ERA.
Last season, everything changed. Severino (age 23) and Ray (25) both rebounded with sub-3.00 ERAs for postseason clubs, were All-Stars and garnered Cy Young Award votes. These are extreme cases, but also evidence of how much can change in a single year, especially for talented young pitchers.
With that in mind, here are five more pitchers -- all age 27 or younger -- who could take a step forward this season. Like Severino and Ray in 2016, each of them is coming off a year in which he underperformed in both his Fielding Independent Pitching (FIP) and Expected Weighted On-base Average (xwOBA) allowed. (The latter metric factors in a pitcher's actual strikeouts and walks, plus the quality of contact made against him, according to Statcast™).
Players are listed with their seasonal ages (as of July 1, 2018).
Matthew Boyd, Tigers, age 27
2017 stats: 135 IP, 5.27 ERA, 4.51 FIP, .357 wOBA, .325 xwOBA
Teammate and fellow left-hander Daniel Norris -- who will compete with Boyd for a rotation spot this spring -- is another candidate to improve significantly upon his 2017 numbers (5.31 ERA). But while Norris gave up lots of hard contact last year, Boyd did the opposite. Of the 99 pitchers who generated at least 400 batted balls, only six allowed a lower rate of hard contact (95-plus mph exit velocity) than Boyd: Lance Lynn, Clayton Kershaw, Giovany Gonzalez, Alex Wood, Jacob deGrom and Corey Kluber. Teammate Michael Fulmer ranked just behind him. That's an accomplished list.
Also encouraging for Boyd is that after he returned from a stint at Triple-A early in the second half, his four-seam fastball velocity rose by nearly 1 mph (up to 92.6 mph), and his strikeout rate jumped from 14.4 percent to 21 percent. He delivered a 2.95 ERA across six September outings, including a near-no-hitter against the White Sox.

Marco Gonzales, Mariners, age 26
2017 stats: 40 IP, 6.08 ERA, 5.06 FIP, .399 wOBA, .336 xwOBA
Gonzales first reached the Majors in June 2014, barely more than a year after the Cardinals drafted him 19th overall out of Gonzaga. But the polished lefty battled injuries the next year before undergoing Tommy John surgery in April 2016. He returned to game action last May, and Seattle acquired him from St. Louis in July.
In a limited sample, Gonzales' big league results last year were uneven, at best. Yet they came with a .383 BABIP, which was the highest for any pitcher who threw more than 30 innings. Most importantly, he will be two years clear of his surgery as the 2018 season begins, and he has reintroduced the cutter he didn't throw in '17 as a precaution. While no conclusions can be drawn from one Cactus League game, Gonzales' spring debut on Sunday against the Dodgers (two hitless innings, four strikeouts) certainly was an encouraging step toward locking down a rotation spot.

Nick Pivetta, Phillies, age 25
2017 stats: 133 IP, 6.02 ERA, 4.87 FIP, .362 wOBA, .326 xwOBA
Acquired from Washington for Jonathan Papelbon in 2015, Pivetta endured a difficult debut season, posting the fifth-highest ERA in the Majors (minimum 120 innings). But the 6-foot-5 righty flashed some intriguing stuff, including a four-seamer with zip (94.5 mph), a high-spin-rate curveball (2,830 RPM) and an effective slider. His 24 percent strikeout rate ranked 30th among those with 120-plus innings.
Pivetta also showed improvement in the second half, even as his ERA soared. His rate of hard-hit liners and fly balls allowed dropped from one of the highest in the league (24.8 percent) to a better-than-average 16.7 percent. Despite that, batters hit .300 against Pivetta after the break, compared with their expected average of .256. With continued refinement of his pitches -- and some better luck -- the Phillies are bullish on Pivetta's chances in 2018.

Jameson Taillon, Pirates, age 26
2017 stats: 133.2 IP, 4.44 ERA, 3.48 FIP, .341 wOBA, .310 xwOBA
The No. 2 overall pick in the 2010 Draft -- sandwiched between Bryce Harper and Manny Machado -- Taillon has traveled a tough road in his pro career. Taillon fought through health issues, including Tommy John surgery, to make the Majors in mid-2016. Then last May he was diagnosed with testicular cancer. The righty not only came back from that scary diagnosis, but he returned to the Pirates' rotation just five weeks after undergoing surgery.
Taillon was well above average at both inducing ground balls and limiting hard contact. As a result, just 4.4 percent of batted balls against him were barrels -- balls with a combination of exit velocity and launch angle ideal for hitters. That tied for the 13th-lowest rate of 139 pitchers who generated at least 300 batted balls. Despite that, Taillon was a victim of an MLB-high .352 BABIP (minimum 120 innings). Still, he rebounded for a September ERA (3.25) that was in line with what he posted prior to surgery (3.31) and over his 18 starts the season before (3.38).

Luke Weaver, Cardinals, age 24
2017 stats: 60.1 IP, 3.88 ERA, 3.17 FIP, .307 wOBA, .286 xwOBA
Weaver posted a 4.23 ERA in his 10 starts last season, but that obscures how effective the righty was in that role. Among all pitchers with at least 50 innings as a starter, Weaver delivered the 12th-lowest FIP, eighth-highest strikeout rate and seventh-highest strikeout-to-walk ratio, as he produced an elite whiffs-per-swing rate with his four-seamer. Among starters who faced at least 200 batters, he tied for the 19th-lowest expected wOBA allowed, edging out teammate Carlos Martinez.
It remains to be seen whether Weaver can maintain this small-sample success as teams get a chance to adjust to him. Developing his curveball as a reliable third pitch would help, as Weaver threw the four-seamer and changeup about 85 percent of the time last year. But the 2014 first-round pick will get a shot at playing a big role in a Cardinals' rotation searching for reliable options behind Martinez.