The Scotland international has become a key player for United over the past 12 months.
Ferguson said: “Darren has matured so much over the last year or so and I think it’s fair to say that not so long ago he wasn’t considered as a regular player.
“That isn’t the case now. At the moment, it is impossible for me not to pick him for any of our big games.
“That’s how influential he has become. His form in the last year has been absolutely magnificent and he seems to perform even better in the big-game situations.
“John O’Shea is the same. The two of them have matured to the level where they are now key players for our club.
“These two players will be the ones that will take over the spirit of the club from players who have been here a long time like Ryan Giggs, Paul Scholes and Gary Neville.
“Manchester United always needs players who are local or home-grown. Fletcher is one of those now and he will be someone who others look up to.”