Whilst I certainly think that we could really need an established and experienced centre back, we wouldn't solve all of our defensive problems by signing one of the big names. We need to defend better as a unit, as defending starts up front and everybody needs to put a shift in. We make it hard for ourselves at times, as we are to rash in certain situations and that applies for everybody, not just the back four. It's more obvious when they make the mistakes and therefore easy for people to say "our defence isn't up to the standard we need at United", but you see teams with a lot less quality defending better, because they defend as a team.
I think that Evans, Jones, Smalling and Rojo all have the quality to be United players, but whilst I'm confident about their ability, we really need to take their injury records into consideration. Evans has been the longest servant of them and as far as I remember without looking statistics up, he has been injured for lenghty periods multiple times over pretty much every season he played for us (please correct me if untrue). The same applies to Jones down to his style of being a lion hearted defender and Smalling also. Evans should be our leading figure at the back considering age, experience and maturity but without him, we have talented but still inexperienced players at leading the back line in Smalling (24) and Jones (22). Rojo (24) just joined us and still has to adapt to the team/league and also has the language barrier, so I wouldn't take him into consideration for that role just yet.
The question I'm asking myself is, how much time should we give someone like Evans for example, who has been at the club for a long period now, to prove that he is physically capable over the course of the season to lead the back four?
I'm really not questioning his ablities, as I really like his style, but we really should think about that and find a solution. We just can't afford to have so many injuries at the back all the time, because we need to get to a certain level of naturalness in defence which only comes with consistency. It is a process, but in order for that process to take place, we need players fit and playing. We are three months into the season now and are still on the same level as we were, when we started pre season.