Since it's a double header, they should have the extra roster spot available. They will probably start someone else, but they can't per the rules use a position player unless it's a blow out late. There's going to be a lot of scrutiny if anything weird happens due to this impacting a team whose season is over, so they will at least try to make it look like a normal game.
Plus I'm sure the Mets would love to be able to bounce the Braves from the post season.
XI Wiki


