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).