Chapter four, part 12, section C of the MLB Rule Book...
A suspended game shall be resumed at the exact point of suspension of the original game. The completion of a suspended game is a continuation of the original game. The lineup and batting order of both teams shall be exactly the same as the lineup and batting order at the moment of suspension, subject to the rules governing substitution. Any player may be replaced by a player who had not been in the game prior to the suspension. No player removed before the suspension may be returned to the lineup.
A player who was not with the club when the game was suspended may be used as a substitute, even if he has taken the place of a player no longer with the club who would not have been eligible because he had been removed from the lineup before the game was suspended.
Rule 4.12(c) Comment: If immediately prior to the call of a suspended game, a substitute pitcher has been announced but has not retired the side or pitched until the batter becomes a baserunner, such pitcher, when the suspended game is later resumed may, but is not required to start the resumed portion of the game. However, if he does not start he will be considered as having been substituted for and may not be used in that game.