The Argentina international midfielder told the Daily Mail: "I arrived with high hopes and a desire to succeed in England but my adaptation was not easy.
"Maybe the transfer fee (£28.1million) affected me, too. Maybe that weighed me down.
"I won a title in 2003 at Old Trafford and that will always be with me, but it was a bittersweet time. And when I moved to Chelsea it was worse.
"Perhaps I was just more suited to Italy and perhaps my style just wasn't right for the Premier League. Sometimes it happens.
"Each year I read in the papers that it will be the last for Ferguson, but I seriously think that he cannot live without football. Ferguson has enormous experience and so many contacts. People respect his work.
"I think when he leaves, United will have a huge problem. I think, for example, that it would be very difficult for Carlos Queiroz to step up and replace him.
"In football today, the words and opinions of Ferguson perhaps carry more weight than any other coach in the game.
"In this sport it is so important that the coach has total confidence in all 11 players and they have confidence in him. I think that is the way it is now at United and that's down to Ferguson.
"And put it this way . . . a controversy between Ferguson and a player is a bad business for the player. I certainly know that."