Postseason roster rules
Teams submit a 25-man roster prior to each round of the postseason comprised of postseason-eligible players. A club may request permission from the Commissioner's Office to replace a player who is injured during the course of a series, but that player is then ineligible for the rest of that round and the subsequent round, if there is one. A pitcher may be replaced only by another pitcher, and a position player only by another position player.
Teams carry extra players throughout the postseason in the event of injuries, and those players, as well as players on the injured list, can be in the dugout during games, within reason.