I'll be honest from what I've seen so far nothing to me screams he's going to be a Utd centre back and by Utd centre back I'm talking Bruce, Pallister, Stam, Vidic, Rio level of quality. Now he might reach the level of decent squad player like Brown, Berg, Silvestre, O'Shea, Evans, Smalling but I don't see him being a nailed on starter in a team that can challenge for titles.
The positive is at least he's starting to show signs of improvement and he still has time on his side to develop. Having a regular partner must be helping him as he's got Smalling alongside of him in every game currently now Smalling is what he is he won't really improve at his age but he's a competent centre back who annoyingly has the odd howler in him every now and again.
I'm a firm believer that centre backs have to be judged as partnerships. So much about defending is working with your team mate and whenever I think of great centre backs I always think of them in pairs. Terry & Carvalho, Rio & Vidic, Ramos & Pique, Chiellini & Bonucci etc you need to find two guys who really compliment each other and Utd just haven't had that.
It seems the manager has some faith in him as the likes of Jones, Rojo & Bailly are all fit now and he's not been tempted to bring any of them in despite the team letting in loads of goals.