Every player has to be on one of the rosters at all times with the exception of Triple-A players who are about to be called up. They can be replaced before the transaction with the big league club officially goes through.
One thing to note: The rosters you see on the team websites are not official. Even the rosters printed out and distributed in the press box by the minor league teams themselves are not the official roster. Only the big league team's player development office can make official roster changes, and their paperwork is not publicly viewable. The teams just pass along the info they get from the player development office when they get it. I have seen cases where a player shows up in a game and no one in the media office of his team was ever told to add him (so no one in the press box knows who he is), yet he was legally able to play that day because the farm director's secretary DID send the paperwork to the Minor League administrative office.
I am not sure which roster Yorman is officially on at the moment, but I would expect you will find out tomorrow.