Sir Alex Ferguson says there is no guarantee Roy Keane will return to football after seeing the former Manchester United captain walk out on his job as Sunderland manager just 48 hours before Saturday's Premier League clash with United at Old Trafford.

Keane's future is now open to debate following his decision to end his two-year reign at Sunderland with the club sitting in the Premier League's bottom three.

And Ferguson, who worked with the fiery Irishman for twelve years at United, admits that Keane might now choose not to return to management following his first attempt at the job at Sunderland.

Ferguson said: "We all wish Roy well because he was a great player here at United and it's a pity that he has left Sunderland. It's difficult to say whether he will want to come back into management or not.

"But there are very few managers that last more than 3-4 years at a club nowadays and the lifespan of a manager at a club is getting shorter and shorter.

"They can move on to different challenges, but it's a very precarious industry nowadays and a very emotional game.

"A couple of years ago, people were saying that I should have gone four years earlier -- the black flags were out! Football people like myself are fortunate to have been at a club for so long, though. I haven't had to go through that emotional situation.

"Roy was an incredible controversial character, though. He always had something to say about the game, even as a player. And he did a good job at Sunderland."

United face Sunderland with Scotland midfielder Darren Fletcher struggling to overcome a hamstring injury. Veteran midfielder Paul Scholes is set for another stint on the bench, however, after returning from a two-month knee injury lay-off in the midweek 5-3 League Cup win against Blackburn.

Argentinian striker Carlos Tevez has been credited with four of United's goals in that quarterfinal victory and, despite his reduced involvement since the September arrival of Dimitar Berbatov, Ferguson insists that he still has a role to play at Old Trafford.

He said: "Without question, Carlos has given me a big selection problem and I have been doodling on my notepad over the last couple of days, trying to work out how I can fit all these players in.

"He gave a fantastic performance of energy, aggression and running. He is always a tryer and I admire him for that, but it doesn't make my job any easier!

"It's obvious that he doesn't enjoy not playing. He wants to play, but do I want players who don't like being left out or do I want players who are happy to sit in the stand?