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.
Ferguson recently said that a nasty leg injury could be a "blessing" for Rooney as it will enable him to improve his fitness in the gym after a sluggish start to the new season.
"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."
- Manchester United