A postseason roster takes effect only if a team clinches a playoff berth. Players who are part of the team 's final roster at the end of the regular season are eligible to participate in the postseason. Any player who has been traded from a different team, spent time in the Minor Leagues, or signed later in the season with the team (no later than August 31) is eligible to participate in the postseason. A postseason roster is allowed up to 25 active players. Other players who are not on the 25 - man active roster will be assigned to the postseason secondary squad. Players who are on the disabled list or any other non-active transaction by the end of the regular season will have their transactions passed on in the postseason. Rosters for a series are set at the beginning of the series and no changes to the 25 - man active roster are allowed except when a player is moved to the disabled list or any other inactive transaction. If a player is moved to the disabled list or another inactive transaction during a series, he then becomes ineligible to be returned to the 25 - man active roster for the remainder of the series as well as the next series if applicable. If any player goes on any inactive transaction, any player from the 40 - man roster can be promoted to the 25 - man active roster for the remainder of the series if applicable.
when do mlb rosters have to be set
at the beginning of the series