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The centre of the Manchester United defence is in a state of flux.

Arguably the greatest central defensive partnership in the club’s history, Rio Ferdinand and Nemanja Vidic, is now in its twilight.

Since 2006, this pair have presided over five Premier League titles and three Champions League finals, winning it in 2008.

But over the last year they have both shown signs of age and rust.

Having turned 35 yesterday, Ferdinand is no longer the player he was, and the simple truth is age has caught up with him.

He can still turn in great performances, but not twice or three times a week now, and when he faces a striker with genuine pace he can suddenly look uncertain and vulnerable.

The ease with which West Brom’s Morgan Amalfitano slipped the ball through his legs en route to scoring in September has come to symbolise his increasing demise.

Nemanja Vidic has only just turned 32, still peak age for a central defender, but the year he recently spent out of the game with his knee injury has affected him.

The Serbian is not finished yet, and will play on for longer than Ferdinand, but could he also be past his absolute best now?

Over the last couple of seasons Phil Jones, Jonny Evans and Chris Smalling have all been auditioning for one of these positions.

Only four months in to the job, David Moyes will now realise he needs to start making the transition from the Vidic-Ferdinand era.

Three into two does not go, so who from Evans, Jones and Smalling should step up and form a partnership? My vote would go to Jones and Smalling.

As I wrote here last week, Phil Jones is not a midfielder; he does not have the technical ability or passing range to prosper there.

Since then I was at Craven Cottage last Saturday to see him play in central midfield again against Fulham.

United comfortably won 3-1, and Jones put in another solid display, but it should only ever be a stop-gap role for him.

With his strength and presence Jones remains destined to move back in to centre of defence which Sir Alex Ferguson, the man who bought him in 2011, believes is his best position.

Alongside him in the future should be Chris Smalling, now in his fourth season at Old Trafford after arriving from Fulham for £12 million.

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It could be argued that a Jones-Smalling pairing is a younger clone of the Vidic-Ferdinand partnership, with Jones replicating Vidic’s dogged determination, while Smalling does a good impression of Ferdinand’s more cultured approach to defending.

So far Smalling has shown only fleeting glimpses of what he can do, but he is always difficult to beat with his size and pace, while he also brings the ball out of defence with confidence.

Jonny Evans is a fine defender whom United fans have learned to trust when his name appears on the team sheet. Last season Ferguson even started him against Real Madrid at the Bernabeu in the Champions League.

But I would suggest this is as good as Evans is going to get, and he has his limitations, being not quite fast enough, too rigid and often prone to mistakes.

At the moment there is nothing between Evans and Smalling, but overall Smalling has the greater potential. A great defender lurks within.

Competition for these places is not restricted to players already on the payroll. Moyes will also look beyond Old Trafford to bolster his defence and has recently been linked with Porto’s Eliaquim Mangala.

The United manager didn’t enter the transfer market for a defender last summer, preferring to take a good look this season at what he already has—and if Jones and Smalling are given the opportunity they could become United’s defensive partnership of the future.