Ferguson also regards David Luiz as a success this season.
"They are in transition," said Ferguson. "Villas-Boas is playing a different way and when we played them earlier in the season they were far more fluid. He's brought in a couple of players who have made a difference in Ramires and Mata and he's given Sturridge a chance and he repaid him with nine goals from the wide right position, which isn't a bad contribution at all.
"Drogba is getting a bit older now and he's had to address that part, letting Anelka go, so there are changes.
"Luiz has become an important figure because he's allowed Alex to leave and Cahill has come in too, so there are changes there. But we've won the last four against them, and we were absolutely brilliant in the league against them last season, so maybe that helps."
