The Bills would have to sign that player directly to the 53. They will elevate Fox to the 53 this week, and Shaq is taking his place on the PS. It's just depth, so we don't have to call up someone off the couch and insert them directly into the game-day lineup. This move demonstrates that, unlike 90% of TBD, the Bills front office has not given up on this season.
BTW, Morgan Fox is a player that actually has snaps and starts in this league that has never played for the Bills. They went out and found him.