If it feels to you as though there are more rookies in Major League Baseball than usual, you're not wrong. Between the rules allowing for expanded rosters this year and the need to restock those rosters because of positive COVID-19 tests as well as higher injury rates than normal, teams already have used 231 pitchers and 133 position players who meet the qualifications to be a rookie -- that is, they entered the season with fewer than 130 career at-bats, 50 career innings pitched or 45 career days of service time prior to Sept. 1 in a given season.
By comparison, teams last year used 270 pitchers and 171 position players who met the qualifications, albeit over a period of games more than three times as long; the pitching total was the highest of at least the past 10 seasons, while the position player total wasn't far off from the decade's high of 181, set in 2011.
Some of these rookies have been standouts, though with about 80% of the 60-game schedule played, the sample sizes are still small, and the separation between the good ones and the very best is narrow. What follows here is one person's attempt to identify the most impactful rookies this year, split between position players and pitchers rather than by league.
For this exercise, I'm guided by but not limited to the FanGraphs version of wins above replacement, and mindful of sample sizes; while some rookies have excelled while getting fewer than 100 plate appearances or throwing fewer than 30 innings, the larger bodies of work carry more weight as far as I'm concerned.