Bell has never looked like a starting LT at any point in his young career, healthy or not. Average in pass protection but quite beatable to the good pass rushers, and a complete mess in the run game. I find him to be one of the more overrated Bills on this forum that there is right now...and he's not even rated all that highly.
Just because he's young and inexperienced, you don't ASSUME he's going to vastly improve. You could put me at left tackle and say, "He has no experience and no conditioning, he'll improve." My improvement would be irrelevant because I do not and never will have the skills to start at left tackle in the NFL. Could I be Demetrius Bell?
Last season was supposed to be the whole new ballgame, after the offseason of pro conditioning and strength training yada yada yada. I'm not holding my breath waiting for this guy to suddenly show competency when he has given us so little reason to believe.