The Royals head into September with their best chance at a post-season since 1985. Accordingly, some of us may be out of date with how Major League Baseball handles roster eligibility in September and the post-season. Let's talk things through.
First things first, the active Major League roster is allowed to expand up to 40 on September 1. Most minor league affiliates will end their regular season shortly after that date, which is why Aaron Crow and Liam Hendriks were sent to AA Northwest Arkansas. Their season ends next week, which allows the Royals to summon those pitchers immediately, rather than spend the usual ten days in the minors following an optional assignment. Had those two been sent to Omaha, they would have been required to spend ten days there, or until Omaha's season is over, and Omaha has a decent shot to make the Pacific Coast League playoffs.
When rosters expand you will start to see teams call up players you've never heard of, many of which you will never see again. To be added to the active roster, you must obviously be added to the 40-man roster. This is what makes it a bit problematic to add pitching phenoms Brandon Finnegan and Christian Binford, since they are not currently on the 40-man roster. Someone has got to go. Bruce Chen, Casey Coleman, and Lane Adams would be prominent candidates.
So who is eligible for the post-season roster? Anyone who is on the active 25-man Major League roster as of midnight on August 31 is eligible for post-season play. So if the Royals trade Aaron Crow to Miami for Giancarlo Stanton on September 1, Stanton would not be allowed to play in the playoffs for the Royals. Also eligible is anyone on the disabled list. So Eric Hosmer is eligible for post-season action even though he's not going to be on the active roster on August 31.
Additionally, a club can add any player from the organization to replace a player on the disabled list. Earlier this week, the Royals called up Michael Mariot and immediately placed him on the 60-day disabled list. While this seems like a curious move, this was done explicitly for the purpose of using Mariot as a placeholder in case the Royals want to add another player to their post-season roster that wasn't on the team August 31. So technically, Brandon Finnegan can be called up September 2 and replace Michael Mariot's "place" on the active roster. This loophole has been exploited by teams for years and was used most famously by the Angels in 2002 with Francisco Rodriguez, who had as many post-season innings that year as he did regular season Major League innings.
During the playoffs, teams can change their playoff rosters before each series. They can select any player under the eligibility requirements listed above. If any player is injured during a series, that player can also be replaced on a roster, although that makes the injured player ineligible for the remainder of that series AND the next series. Major League Baseball once required that an injured pitcher be replaced by a pitcher, but that is no longer the case.
(Special thanks to Bless You Boys, for their helpful roster eligibility primer from 2013).