Okay ... this just in from the rules expert, spycake ... "NOTE: A player has up to 72 hours to report after being assigned from one MLB club to another. The player counts against the club's MLB Reserve List (40-man roster) as soon as the trade is filed with the MLB Commissioner, but the player does not count against the club's MLB Active List (25-man roster) until he physically reports to the club and is in uniform." https://www.thecubreporter.com/book/export/html/3506 And this.