From the manual:

Play-off Rosters:

Active rosters for the play-offs will be set to 25. No roster moves are permitted during a play-off series (they are turned off), but play-off teams may make roster moves between series. Batters with less than 50 plate appearances in the corresponding Major League season or pitchers with less than 20 innings pitched in the corresponding Major League season are INELIGIBLE for the play-offs. The system will automatically cut any such player from your roster before the play-offs and the offending team will have to compete with 24 players.
Please refer to the USAGE/LIMITATIONS section of GAME INFO for more on play-off rosters