Manchester United boss Sir Alex Ferguson doesn't believe Roy Keane will return to football after quitting Sunderland this week.

Fergie said: "It's difficult to say whether he'll be back.

"There are very few managers that last more than three or four years at a club now. It's getting shorter and shorter, the lifespan of a manager at a club. They can move on of course to other jobs and challenges but it's a very precarious industry.

"Football people like myself are fortunate that we have been so long at one club. We haven't had to go through that emotional turmoil. But it always happens at Christmas time, doesn't it? It's horrible."

Ferguson believes the pressure put on managers and chairmen by fans is forcing the constant changes.

He said: "It's to do with the emotions in the game. It's greater than ever and crowd participation has increased in terms of volatility.

"I've seen a photograph of United and Leeds players all scrapping in the middle of the pitch, Jackie Charlton, Norman Hunter - all these warriors going in there - Nobby Stiles, Paddy Crerand.

"But look at the crowd and there is not a bit of emotion. They are just watching it as a spectacle.

"Today you wouldn't get that. They would all be screaming blue murder. So you are in a very emotional industry too and those emotions can get to everyone. If you lose two or three games in today's world, you could be in trouble.

"We just wish Roy well because he was a great player here. It's a pity.

"He was an incredible, controversial character and he always had something to say about the game. He was very controversial of course even as a player. He's quite an interesting character."