Sir Alex Ferguson didn't get an awful lot wrong during his time at Manchester United but, three-and-half years after he said it, his assertion that Jonny Evans and Chris Smalling would be Old Trafford's central defensive pairing of the future remains distinctly unproven.

'That is how I look upon it,' said Ferguson in an interview with the official Inside United magazine in February 2011. 'I think their time will come.'

Had things gone as planned, this indeed would be the era of Evans and Smalling. Both have been at the club for long enough now. This season, for the first time, the path to the first team has been left clear by the departures of Rio Ferdinand and Nemanja Vidic.

Chris Smalling has failed to cement a place in the heart of Manchester United's defence

Jonny Evans had to be replaced during the first-half against Leicester with an injury

Sir Alex Ferguson (centre right) predicted Smalling (far right) and Evans would be the club's defensive pairing

Still United wait, though. As both careers continues to be hampered by injury, Evans and Smalling remain some way short of a pairing that can move United forwards to where they need to be in the coming weeks, months and years.

Recent problems for Louis van Gaal in his first season at United have only highlighted the issue. Four goals were conceded at Milton Keynes Dons in the Capital One Cup while an astonishing five were shipped at Leicester in the Barclays Premier League at the weekend.

Now, with Evans injured and young prospect Tyler Blackett suspended after a red card on Sunday, Van Gaal is having to countenance the prospect of using central midfielder Michael Carrick as a defender when he returns from the ankle injury that has ruined his summer and early season.

Certainly, it is difficult to see who United's central defensive pairing will be this time next year, never mind in five years' time. This, as much as anything, illustrates how the natural evolution that top clubs like to oversee in all areas of a playing squad has stalled in defensive areas at Old Trafford in recent years.

Evans' latest injury has added to Louis van Gaal's central defensive crisis at the club

Smalling reacts as United capitulate in their 5-3 Premier League defeat at Leicester on Sunday

Midfielder Michael Carrick could be deployed at centre back for United when he returns from injury

Carrick has missed the beginning of the campaign after undergoing ankle surgery during pre-season

England intetnational Phil Jones (left) has failed to make a consistent impact at Old Trafford

NO CONSISTENCY Highest number of consecutive league matches played by Evans, Smalling and Jones in last two seasons... Evans - Seven Smalling - 12 Jones - 11 Advertisement

Ferguson also signed England man Phil Jones from Blackburn three summers ago. That was part of the grand plan, too. But Jones, shunted about too often from midfield to defence, has similarly struggled to consistently make an impact.

Neither of the three players will like to hear it, no player does, but Jones, Evans and Smalling are all too injury prone. That, in some way, is why it has proved so difficult to judge them. Certainly in the case of Smalling and Jones it is difficult to judge or predict how effective a United player they will be simply because we haven't seen enough of them.

When you consider the fact that Evans (7), Jones (11) and Smalling (12) have managed to pay so few consecutive league games over the course of the last two seasons then the issue with fitness becomes clear. There is not a young footballer in the world who will improve if his body doesn't allow him to play.

On Sunday at Leicester, for example, United arrived in the East Midlands with Evans fit, Jones injured and Smalling on the bench after a month out with a groin problem. By full-time, Evans was in a cast with a foot injury and Smalling had come on to replace him. Blackett, meanwhile, had been sent off. And so it goes on.

Tyler Blackett (right) was sent off for United during their humbling at the King Power Stadium

Evans (centre) has only managed to play seven games in a row for United in the last two years

Interestingly, United's medical staff began this season convinced that their defenders would enjoy relatively injury-free campaigns. With players like the three we have mentioned reaching physical maturity, it was anticipated inside the club that they would be available more often.

How you can make those assessments is hard to understand but, whatever the science behind it, the opposite is proving true so far.

Smalling and Jones are only 24 and 22, of course. They should still have their best years ahead of them. Evans, meanwhile, is 26 and has shown enough, when fit, to suggest that he has the potential at least to be a United player of some stature.

As time moves on, though, so must the careers of these three players.

As Van Gaal has shown so far during his time at the club, if he feels players aren't going to be of sufficient use to him, he will just go and buy somebody else.