Question: how do mlb playoff rosters work?
Answer: Players must be on a club's roster by Sept. 15 in order to be eligible for postseason play. ... (For example, a player on the 10-day injured list who has been on it for at least 10 days, or a player who has been on the 60-day injured list for at least 60 days.) Players who are acquired in September or after are ineligible.