Manchester United striker Wayne Rooney aware fitness issues can lead to wrath of Alex Ferguson

Wayne Rooney is out for a month – time he needs to spend getting fit (Getty Images)

Manchester United's Wayne Rooney knows he has to be at the top of his game or risk the wrath of manager Sir Alex Ferguson.

The England forward admits that he put on seven pounds during the off-season but explained the hard work he puts in during training makes the excess weight easy to shift.

"As a striker I need to work hard all the time. I need to be sharp, which means my fitness has to be right to play well. If it isn't, it shows," he told the Mirror, who are serialising his latest autobiography.

"It would probably be different if I were a full-back. I could hide a bit, make fewer runs into the opposition half and get away with it.

"As a centre-forward for Manchester United, there's no place to hide.

"I've got to work as hard as I can, otherwise the manager will haul me off the pitch or drop me for the next game.

"There's no room for failure or second best at this club."

Rooney found that out late last year after he was dropped for a match against Blackburn at Old Trafford.

Ferguson was unhappy after it came to light Rooney had been out socialising after a game.

"I'm happy at Manchester United, despite the downs that sometimes take place at a football club," he continued.

"Like when we stuff Wigan 5-0 on Boxing Day.

"I go out for dinner with a few of the lads, and our other halves, to a hotel.

"The next day, the manager pulls me up and tells me he's not happy and doesn't feel I've trained properly.

"He fines me, but there's worse to come. I'm dropped for the next game, on New Year's Eve, against Blackburn.

"At a lot of clubs, people wouldn't bat an eyelid at players having a night out six days before a game. But that's the difference at Manchester United and a mark of the high standards the manager demands.

"It's a big deal, another lesson learned."

And Rooney also knows his body is not as durable as it used to be.

"Physically I've taken a bit of a battering over the years; being lumped by Transformer-sized centre-backs or having my muscles smashed by falls, shoulder barges and last-ditch tackles, day in, day out, has left me a bit bruised," Rooney, currently sidelined for a month after having his leg wounded during a game with Fulham, said.

"When I get up in the morning after a game, I struggle to walk for the first half an hour. I ache a bit. It wasn't like that when I was a lad.

"I remember sometimes when I finished training or playing with Everton and United, I'd want to play some more. But football has had a massive impact on my body because my game is based on speed, power and intensity.

"Like any player I'm fearful of getting a career-ending injury. I could be in the best form of my life and then one day a bad tackle might finish my time in the sport. It's over then.

"But that's the risk I take as a player in every match. I know football is such a short career that one day, at any age, the game could be snatched from me unexpectedly.

"So I want to decide when I leave football, not a physio, or an opponent's boot."

PA