Manchester United haven't started the campaign in sparkling form so far this season, already losing two of their opening seven league fixtures, with the main cause for concern stemming from the side's lack of form, organisation and relative fitness at the back. Should this continue to remain a weak area in the squad, will manager Sir Alex Ferguson be forced to bring in a number of new faces?

In their 10 games so far this season, the side have kept just three clean sheets, against Galatasaray, Wigan and Newcastle, going on to concede 13 goals in the process, with the goalkeeping selection policy involving David De Gea and Anders Lindegaard hardly helping matters when it comes to a settled back five. The stat that they've fallen behind in six of their 10 games so far is a shocking one and while the firepower they boast is exceptional, as they showed against Tottenham, they can;'t continue to keep giving teams head starts.

Nemanja Vidic, Chris Smalling and Phil Jones have all had to endure and still are, spells on the sidelines through injury and it's become something of a regular occurrence to see Ferguson field a makeshift back four with Michael Carrick in it, plugging holes were a team of its resources simply shouldn't have them and it seems that every season that this same problem keeps happening.

It's not just the injury-prone nature of some of the players in question that is somewhat alarming, though, rather the age of some of the team's first-choice back four, with the likes of Patrice Evra (31), Rio Ferdinand (33) and Nemanja Vidic (30) entering the latter stages of their respective careers, with the Serbian skipper in particular looking rusty on his comeback due in part to having apparently never fully recovered from his injury.

While Smalling remains a long-term prospect for the club, having made just 35 league appearances in two and a bit seasons isn't the greatest record around and he seems to have been seriously restricted by his penchant for the treatment table, while Jones lacks the positional discipline at present to be anything more than a liability in defence and it seems as if Ferguson doesn't quite trust him there yet fully.

One shining light this term has been the form of Brazilian right-back Rafael, who appears to finally be growing into his role more and more, offering a threat going forward while combining it with his tenacious defensive play. At 22 years old, along with Smalling and Jones, he seems to be one of the only defenders that could enter his peak over the course of the next few years.

While much has been made of Ferdinand's decline, perhaps too much, there's no denying that he isn't quite the force of old, when at the peak of his powers he was a truly world-class operator and classy technician, but he's incapable of performing to a consistently high standard as often these days. He's not as injury-prone as his detractors will have you believe but he's a declining force.

Evra's own slump in form has now become the norm and there looks to be no return for him; the sheer amount of attacks and goals that come from his side of the pitch is shocking and he seems to think dangling your leg to try and stop an incoming cross is sufficient enough - he's got complacent and dare I say it, lazy in terms of his work-rate and ability to close the ball down. He was always susceptible to pace and the ball in behind, but the bad far outweighs the good these days.

In terms of quality, this isn't the finest of eras when it comes to defenders in the Premier League or indeed world football; Barcelona routinely play Alex Song and Javier Mascherano as their central defensive partnership this season due to injuries, while Bayern Munich, Real Madrid and Manchester City are hardly watertight at the back either. To put it simply, you can win silverware without having the stable platform that was once a necessity, so having a patchy back four doesn't mean that United are suddenly incapable of winning a trophy this season.

However, when it comes to individual form, how they work as a collective and their respective ages, it looks as if Ferguson is faced with a dilemma on the horizon about how to go about reshaping his back four. Alexander Buttner has already been brought in to challenge Evra for a starting spot, while Jonny Evans will remain the cornerstone of the back four for years to come, but there's not an awful lot in terms of depth at the moment and that will only get worse as time goes on and the wily old Scot may have to start blooding in the likes of Michael Keane, Scott Wootton, Frederic Veseli and Marnick Vermijl in the league sooner than he would have liked.

It's far from a crisis at the moment and they probably have just about enough to get them through this season, but there's only so long that you can continue to blame injuries for your poor defensive performances. United's decline at the back has directly coincided with key figures such as Evra, Ferdinand and Vidic struggling with their own form and fitness and eventually Ferguson will have to look to the future and consider seriously replacing, because what's the point of having a first-choice back four if they're never all fit together at the same time? In the modern game you need more depth than that and the squad will need re-shaping in the not too distant future.

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