<P> On September 1, the Major League team's roster expands from the 25 - man active roster to the entire 40 - man roster . At this point, any player on the expanded roster on August 31 may play for the Major League team . </P> <P> September call - ups are players from the minors who are playing in September to get Major League experience and, especially for teams in contention, to provide reinforcements down the stretch . Note, however, that once the roster expands, in practice teams will utilize a number of September call - ups (typically up to six), but seldom if ever the full 15 extra players that the rules allow . </P> <P> 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 . </P> <P> For a player to be eligible for the postseason active roster, he must have either been on his club's expanded roster or disabled list as of midnight ET on August 31 of that year and not placed on the 60 day disabled list after August 1 . The one exception is for replacing players on the disabled list . Since the 2014 season, an injured player eligible for postseason play may be replaced by any player within his club's organization, including players assigned to the club's minor league affiliates who are not on the major league 40 - man roster . Prior to 2014, only players who were on a club's 40 - man roster (including those added to the expanded roster after August 31 who would not otherwise be eligible for the postseason) at the conclusion of the regular season were eligible to replace injured players on postseason rosters . The 2015 Kansas City Royals were the first club to exploit this new provision when they added Raúl A. Mondesí--who at the time was assigned to the Royals' Double - A affiliate and was not on the 40 - man roster--to their World Series roster . </P>

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