Rooney, who has been ruled out of England's World Cup qualifier against Moldova in Chisinau on Friday because he is recovering from a nasty gash to his thigh, made the comment in his latest autobiography, which is being serialised by the Daily Mirror.
In the book he refers to a statement he issued in October 2010 in which he questioned United's ability to attract top players, and indicated that he wanted to leave Old Trafford because the club could not meet his ambitions.
However, two days later he changed his mind and signed a new five-year contract, but said he now looks back on the incident with regret.
"In September 2010 my ankle puts me on the sidelines.
"I get frustrated with myself, my game, my injury, and everything around me.
"I'm stuck in a cycle of bad form but I can't get out of it.
"And that's when I make the biggest mistake of my football career.
"In October, I release a statement which publicly questions my happiness at Old Trafford. Am I better off elsewhere?
"Everyone makes a fuss. There are discussions inside United to sort out the issue, people outside United chuck their opinions around, but the thing is, nobody really knows what's going on in my life.
"None of them understand where I am in my career. They don't know where my head's at.
"The only person who really knows what's going on in there is me, but even I'm not sure what I want.
"Then the manager has his say. 'Sometimes you look in a field and you see a cow and you think it's a better cow than the one you have in your own field. And it never really works that way'.
"He's saying the grass isn't always greener, and he's right.
"I like what's in my field. I'm wrong. United want the same as me: trophies, success, to be the best."
The 26-year-old told Sky Sports he hoped to be back in action for United later in the month, following the gash he suffered after coming on as a substitute against Fulham a fortnight ago.
