Speaking to L'Equipe, Ferguson said Bayern Munich's Oliver Kahn and Parma shot-stopper Gianluigi Buffon were the only other keepers he was interested in being long-term successors for Peter Schmeichel.
"I followed him for several years and I know almost everything about him," Ferguson said.
"Fabien is in a class of his own and that's also why I got him to get him here. Manchester needed a great keeper.
"For me, a great keeper must have his mood. Fabien is the worthy successor of Peter Schmeichel."
The Red Devils boss said his only worry about the keeper is his keenness for playing outfield in training.
"Fabien has already injured himself twice by tackling like a fool," Ferguson added. "Sometimes I need to hold him back. He is a man of extremes. But I do not angry with him."
"In my eyes, only three keepers could have strengthened our team.
"Barthez, Kahn and Buffon. When I signed Fabien, I knew I didn't make a mistake. Despite this I am still surprised with the guy.
"As I expected he has made some excellent saves but his contribution to the attacking game of the team is unquestionable.
"His throwing are examples of precision and speed. It's a godsend to have a keeper so lucid.
"Giggs and Beckham have found someone quicker. Aside from that, Fabien is perfect.
"He has a technique worthy of an outfield field player."
"The more important the match is important, the more I feel he will come into his own.
"This guy doesn't know fear, nor stress. He has a sane approach to the job. Fabien has straight away integrated the United values. He felt quickly at home in Manchester.
"Fabien is a man of the people. This town and this club, it's the anti-Monaco. You need a great motivation to sign here."
"The coach lets me live as I want. But I am sure he knows everything about me. Anyway he never complained to me about what I do outside the job.
"There is so much trust in me here that I never feel as free as when I come to the Stadium. They respect me for what I am. I like this culture. They could try to give me advice, but they know I will not listen to it. I am an instinctive player."
Ferguson admitted that he knows about Barthez's smoking habit and said although this is unusual in the Premiership, he prefers it to the more familiar poison of English players - alcohol.
"I know Fabien is smoking. I knew that before he came here. Before him, there were just Jesper Olsen and Les Sealey. In England it's rare, but I prefer that than an alcoholic. For the English players, the relationship with alcohol is a true problem.
"Fabien is breathing United. He integrated the relaxed atmosphere and the desire to win of his team-mates. With all he has won during his career, we could have expected him to be a little waned, but it's the contrary. He understood this club has a soul.
"He succeeded in six months what Peter Schmeichel took two years to take on board."
