Fans often are confused about which players are eligible for the postseason roster. Baseball actually changed the rule slightly this year, and it's worth noting.
In the past, players on the 25-man roster as of Aug. 31 were eligible, along with players on the disabled list. The overall restriction was not that severe, however; any player in the organization could replace an injured player.
The difference now is that all players on the 40-man roster as of Aug. 31 are eligible. The Pirates, for example, can name outfielder Gregory Polanco to their postseason roster even though they will not recall him from the minors until Tuesday.
Polanco also was eligible under the previous rule, as a potential injury replacement. But this system is cleaner. Teams did not need to place players on the "phantom" DL in late August to keep them eligible for the postseason. They can simply pick from their 40-man roster before the playoffs start.
Also, there can be no shenanigans with bumping players off the 60-day DL to create space on the 40-man roster. That player cannot be replaced on the 40-man unless he has served the full 60 regular-season days on the DL.