United face Chelsea at Stamford Bridge on Sunday.
Fergie believes Ancelotti has done a great during his short time at the helm at Stamford Bridge.
Fergie said: “Carlo Ancelotti was a great coach at AC Milan, and a great player also, and brought a wealth of experience and knowledge with him.
“I thought he would do well but the change he has brought is a different system to what Chelsea used in the past and it seems to have worked well.
“They have a very, very experienced team and when you’ve got experienced players maybe it is easier to adapt because, generally, they have more tactical knowledge than younger players. That’s maybe been a bonus to them.
“The two years Chelsea won the league [2005 and 2006], they got off to great starts and caught us all on the hop.
“We had to change our ideas about pre-season training because they had such good starts.”
