As a 23-year-old third baseman light on defensive skill and coming off an 80-game suspension for performance-enhancing drugs, Michael Chavis didn't figure heavily in the Boston Red Sox's preseason plans. But desperation -- and the trend toward de-emphasizing infield defense in an age of perpetually rising strikeout rates and analytically assisted positioning -- led them to try the 5-foot-10, 216-pound fireplug at second base when they recalled him in late April. The defending world champions were just 6-13 at the time, and for the second year in a row had a Dustin Pedroia-sized hole at second base. The replacements they could muster were hitting .136/.186/.152. Boston plugged Chavis into that hole, and all he has done since is rank second on the team in slugging percentage (.558) while the Red Sox have gone 20-10.
That kind of impact is something a contender can only hope for when calling up a youngster. Even the best prospect might falter in his first taste of big league action, or take time to find his footing -- far more than the two weeks it took Vladimir Guerrero Jr., sometimes. The 20-year-old consensus No. 1 prospect hit just .146/.222/.171 without a homer through his first 11 games, then .370/.469/.852 with four homers in his next eight.
Guerrero's Toronto Blue Jays don't figure to contend for a playoff spot. What follows here are eight rookies -- some of whom have only just recently arrived, or seen limited MLB action, while others have yet to be called up -- who could play a significant part in their teams' push for a postseason berth. Note that I'm not covering Rookie of the Year candidates who have been around since Opening Day, such as the Mets' Pete Alonso or the Padres' Chris Paddack -- though bless their respective teams for not gaming their service-time clocks -- nor is the primary concern here long-term potential (that's what top-100 lists are for).
2B Keston Hiura, Milwaukee Brewers
Considered by many to be the best collegiate bat available in the 2017 draft, when he was chosen ninth overall, Hiura has continued to mash during his brief professional career. He was hitting .333/.408/.698 with 11 homers in 37 games at Triple-A San Antonio when he was recalled May 14 to replace Travis Shaw, who had strained his right wrist. Hiura's first seven games have included four multihit outings as well as his first homer, off the Braves' Mike Foltynewicz. With a compact, powerful swing, he projects to hit for high average with above-average power, though he's nothing special defensively. To be fair, neither is Mike Moustakas, who was playing second base until Shaw's injury allowed him to return to third. Though Shaw is about to begin a rehab assignment, his struggles (.163/.266/.281) and the Brewers' need for a productive right-handed bat should buy time for Hiura to get a longer look.
1B Nate Lowe, Tampa Bay Rays
Not to be confused with center field prospect Josh Lowe (his younger brother) or second baseman Brandon Lowe (no relation, and pronounced to rhyme with "now"), this Lowe debuted at the end of April but played just nine games before being returned to Triple-A Durham once Austin Meadows returned from the injured list. A 6-foot-4, 245-pound lefty with plus power, an advanced feel for the strike zone, and a decent defender, he's viewed as the Rays' eventual every-day first baseman, though with just seven homers in 238 plate appearances at Durham over two seasons, he might need to show more power before he can move up for good. In the meantime, the team already has a lefty-swinging first baseman in Ji-Man Choi, but he's currently hitting just .264/.336/.403 with three homers and a 106 wRC+, and isn't as good defensively.
LHP Jesus Luzardo, Oakland Athletics
This one's a bit of a dart throw, as the 21-year-old Luzardo is working his way back from a rotator cuff strain suffered in late March, when he was still in the running to break camp with the A's. He has begun throwing to hitters again, and if everything progresses without setback, could debut before the All-Star break. He sports a 92-95 mph fastball that can touch 98, his changeup is considered by some to be a plus pitch, and he's got a plus curveball as well, all thrown with exceptional command.
The A's also have another high-impact starter who could help during the second half in A.J. Puk, a 6-foot-7 southpaw who's recovering from April 2018 Tommy John surgery and is at the stage of throwing simulated games, with a minor league assignment possible next month.
LF/3B Austin Riley, Atlanta Braves
Though regularly a third baseman, the 22-year-old Riley has played primarily left field since debuting May 15, with Ronald Acuna Jr. sliding over to center in place of the injured Ender Inciarte. Riley made an instant impression in his debut, homering off the Cardinals' Michael Wacha in his second plate appearance. Riley has added three more homers and is hitting a sizzling .367/.406/.800 through his first eight games.
Riley's calling card is plus-plus power; he strikes out enough for that to be a concern, though he had cut his Triple-A strikeout rate from 29.3 percent last year to 19.1 percent before his call-up. He had only four games of professional outfield experience before his recall, and in the long run should return to third base, where he has worked hard to become an average-to-plus defender. Given that Inciarte's back stiffness has yet to allow him to return to baseball activity, Riley should be around for a while, and he also can provide insurance if Josh Donaldson lives up to his injury-prone billing.
2B Brendan Rodgers, Colorado Rockies
Given their back-to-back National League wild-card appearances, I'm giving the Rockies (22-25) the benefit of the doubt by considering them contenders. One reason -- albeit not the only one -- Colorado has struggled is the play of its second basemen, who have combined to hit .203/.263/.285 for a 33 wRC+ and minus-0.9 WAR. For that reason, the Rockies just promoted the 22-year-old Rodgers, the third overall pick of the 2015 draft and a shortstop by trade, but one blocked by Trevor Story; he's probably a better long-term fit at second base anyway. Rodgers should be able to hit for average and power, albeit with an aggressive approach that might limit his walk rate.
CF Nick Senzel, Cincinnati Reds
On the surface it would seem to be a stretch to call the Reds contenders, but they entered Thursday with the NL's fourth-best run differential and a 27-22 Pythagorean expected record. Hence the inclusion of Senzel, a 23-year-old consensus top-10 prospect who has hit .240/.321/.427 since debuting May 3. The second overall pick of the 2016 draft out of the University of Tennessee, Senzel played third base in his first two professional seasons. But he moved to second after Eugenio Suarez staked his claim on the hot corner, and then -- after injuries limited him to just 44 games in 2018 -- to center field after Scooter Gennett mashed his way into the Reds' longer-term plans at second. Though he's a work in progress in center, his bat will play anywhere. As Keith Law wrote in January, "He's more likely to hit .320 with a slew of doubles than to end up becoming a 35-homer guy."
OF Kyle Tucker, Houston Astros
Ranked anywhere from eighth to 17th on the various major prospect lists and viewed as an eventual middle-of-the-order bat, the 22-year-old Tucker has already seen major league action -- he hit a dreadful .141/.236/.203 in 72 plate appearances last year. But nobody in the game feels that's representative of his talent, particularly when his Statcast numbers suggest he was getting robbed regularly. He's an all-fields hitter who is realizing his plus power in games, as his 36 homers in 141 games at Triple-A (2018-19) suggest. Capable of playing either outfield corner as well as center, he'll give the Astros an extra bat to help cover for their subpar production at designated hitter and first base, a position left fielder Michael Brantley and right fielder Josh Reddick both worked on adding to their repertoires this spring.
The Astros' top prospect, 21-year-old right-hander Forrest Whitley, could have an impact this season as well, but as he's currently carrying a 10.48 ERA while allowing 2.8 homers per nine innings in Triple-A, he'll have to straighten himself out before he becomes an option.
2B Luis Urias, San Diego Padres: Like Tucker, the 21-year-old Urias -- who ranked 56th on ESPN's top-100 list, and 24th on FanGraphs' -- has struggled mightily in limited big league exposure (.167/.256/.278 in 82 PA scattered between last September and this April), to the point of being sent back to Triple-A El Paso. But with 36-year-old Ian Kinsler manning second in San Diego and looking like a player on his last legs while Urias lights up the Pacific Coast League (.350/.441/.764 with 12 homers in 30 games), that might change soon. The 5-foot-9, 185-pound Urias isn't likely to reproduce that power at the big league level; he's more of a Cesar Hernandez type who will hit for average with gap power while playing a capable second base.
Players listed alphabetically.