Fergie: Time will tell on Vidic.

Sir Alex Ferguson admits doubts will remain over Nemanja Vidic's fitness until the Manchester United defender proves he has fully recovered.

The Serbian is on the comeback trail after two lots of knee surgery.

Vidic has played just 13 times since injuring his calf in the opening Premier League game of last season at West Brom.

After spending two months on the sidelines, Vidic was only eight games into his comeback when he ruptured his cruciate ligaments during a Champions League game in Basle that ended his entire campaign.

The 31-year-old resisted the temptation to rush back earlier than expected so he would be fully recovered in time for pre-season.

However, just five games in, he reported discomfort in his knee which condemned Vidic to undergo more surgery.

Now the Serbian is back for tomorrow's Premier League encounter with Sunderland.

But, recalling the nightmare Gary Neville had after having his ankle broken against Bolton in 2007, Ferguson accepts Vidic's fitness cannot be taken for granted just yet.

"Getting an injury is an issue for players going into their late 20s and early 30s because they start to wonder how many years they've got left," said Ferguson.

"For instance, Gary Neville was 32 when he got his injury and he never really recovered from it.

"He had recurring injuries - it was his calf, then it was his groin, then it was his hamstring, his ankle, it was just a catalogue of injury after injury simply because at that age recovery is a bit more difficult."

Ferguson has yet to decide whether Vidic will start against Sunderland tomorrow.

However, with the Christmas campaign looming, there are lots of matches for the defender to get involved in.

And Ferguson has no doubt the former Red Star Belgrade man can make a positive contribution.

"We hope we've given him the proper rehabilitation," said Ferguson.

"He's not had a lot of football so it is an issue, but we're hoping he sails through.

"There will be opportunities for him because he brings that great experience, that warrior type of defending he's for.

"I don't know whether I will start him tomorrow but at least we have him back and given the number of injuries we've had to defenders over the last two or three years, it's a bonus for us."