That's the number Keith Bulluck has been assigned. Used to belong to a fellow named Harry Carson, whose bust you can see in the Hall of Fame in Canton.
Don't read a great deal into that. Not that Bulluck isn't a great player, but the last guy to wear 53 was rookie MLB Phillip Dillard, who will now be re-assigned another number. Bulluck wore 53 with the Titans, so he really hasn't ventured too far from home, right?