Some players arrive in the big leagues and perform at a star level immediately.

There have been several of those lately. Think Ronald Acuña Jr., or Juan Soto, or Pete Alonso, in this golden age for young hitters.

That can make it easy to forget that, well, sometimes it takes a while. Baseball is hard. Growing pains at the big league level are to be expected, even if a select few manage to avoid them. With a little patience, however, the breakout can be worth the wait.

To highlight these breakout players, MLB.com created a pool of candidates. Players had to be age 26 or younger this season, have exhausted their rookie eligibility prior to 2019, and received significant playing time in both 2018 and ‘19, while improving their OPS by at least 100 points (for hitters), or their ERA by at least a run (for pitchers).

While players such as the Dodgers’ Cody Bellinger and the Reds’ Luis Castillo meet all of those criteria, those who already had performed at a superstar level in the past -- as those two did as rookies in 2017 -- were not considered.

From those remaining, here are the seven most impressive breakouts of this season.

1) Rafael Devers, 3B, Red Sox
That Devers was a league-average hitter in the big leagues at ages 20-21 from 2017-18 was plenty impressive. Still, his numbers slipped from his rookie season to ‘18, when he smacked 21 homers but hit just .240 with a sub-.300 OBP. If that caused any doubts about his potential, Devers has erased them with his performance in ‘19. All he’s doing is leading the American League in hits (167), doubles (46), total bases (300), and RBIs (101), while ranking near the top in average (.322) and OPS (.976). Nobody in the Majors has hit the ball hard (95+ mph) more times, according to Statcast, as Devers has managed to increase both the quantity and quality of his contact.

2) Lucas Giolito, SP, White Sox
A first-round pick and much-hyped top prospect, Giolito faced great expectations. But his first full Major League season in 2018 yielded an MLB-high 6.13 ERA, making him the first qualified pitcher in 10 years to top the 6.00 mark. Nothing in Giolito’s peripheral numbers suggested a massive improvement was coming, but the right-hander labored hard on an offseason overhaul. It worked, to say the least. Giolito, who turned 25 in July, earned his first All-Star selection and has gone 13-6 with a 3.41 ERA in 24 outings. With increased velocity, changes to his pitch mix, and other adjustments, Giolito has gone from second-to-last to tied for fifth among qualified pitchers in strikeout rate, and now looks like the ace Chicago envisioned when it acquired him from Washington.

3) Ketel Marte , 2B/CF, D-backs
He may not get a lot of national recognition, but Marte has absolutely put himself in position to receive significant National League MVP votes behind the big two of Bellinger and Christian Yelich. Those are the only NL position players with a higher FanGraphs WAR than Marte (5.5 entering Monday), who is hitting .321/.383/.574 with 26 homers while leading the NL in hits (153) -- all while splitting time between multiple middle-of-the-diamond positions. This is actually the third consecutive season in which the 25-year-old switch-hitter has made significant gains at the plate, dating back to when he posted a .610 OPS and hit just one homer in 119 games in 2016. Even last year, Marte was about a league-average hitter. But he has continued to make harder contact while also lifting the ball in the air more, and the results have been spectacular.

4) Josh Bell, 1B, Pirates
The switch-hitter hasn’t been the same in the second half, but he went into the All-Star break with a sensational .302/.376/.648 slash line. Bell also became the eighth player to reach 60 extra-base hits in the first half. That signaled the arrival of a 6-foot-4, 240-pound first baseman who despite his strength and imposing frame slugged a modest .436 with 41 big flies over 352 games from 2016-18. Bell has made much harder contact this season, and despite the recent slump, leads the NL in doubles (36) and is tied for first in RBIs (98). Only Alonso has hit more home runs of 440 feet or longer (seven), according to Statcast.

5) Shane Bieber, SP, Indians
Bieber put his breakout on national display when he took All-Star Game MVP honors by striking out the side in relief. It was a representative performance for the 24-year-old righty, who along with teammate Mike Clevinger, is pitching like an ace in 2019. Bieber showed plenty of promise as a rookie last year, with more than a strikeout per inning and a K-to-BB ratio of better than 5-to-1. But he also posted a 4.55 ERA and allowed more than 10 hits per nine innings. Some of that may have been bad luck (see his 3.23 FIP), but Bieber made some significant adjustments, heavily increasing his breaking ball usage and going out of the zone more. Now he enters his start against the Mets on Tuesday with a 3.27 ERA and 200 strikeouts, and is the only MLB pitcher with multiple shutouts.

6) Yoán Moncada, 3B, White Sox
Currently on a Minor League rehab assignment for a hamstring injury that has held him out since the end of July, Moncada is expected to return this week. The White Sox hope he will pick up where he left off, batting .301/.358/.535 with 20 homers and 4.0 WAR while pairing with another breakout infielder, shortstop Tim Anderson. It’s the type of performance that has been expected of the 24-year-old switch-hitter since he was a hot prospect back in Cuba and ultimately signed with the Red Sox for a record-setting bonus in 2015. Traded to Chicago in the Chris Sale deal, Moncada hit below a league-average level over the past two seasons and led the Majors last year with 217 strikeouts. He has cut down on the Ks in ‘19 while also ranking in the top five in the Majors in average exit velocity (93 mph).

7) Max Kepler, OF, Twins
Kepler’s first three big league seasons were remarkably consistent. Each year from 2016-18, he posted an OPS+ between 95-98 and hit 17-20 homers. That was enough to make Kepler a useful big leaguer, but not exactly one of great impact. But a full decade after the Twins signed Kepler out of Germany, the 26-year-old has blossomed. His slugging percentage (.529) is up more than 100 points, his OPS+ has climbed to 125, and he ranks third in the AL with 33 homers. Kepler is hitting lefties as well as righties and has thrived while leading off for a powerful Twins lineup that includes fellow breakout hitters Byron Buxton (when healthy) and Jorge Polanco.