Manchester United boss Sir Alex Ferguson says Chelsea's success this season is down to the tweaking of Carlo Ancelotti.
United travel to the Premiership leaders this weekend.
And Ferguson told Inside United: "Their consistency, compared to last season, has probably been a bit better under Ancelotti.
"He's brought in the system he used at AC Milan of the diamond in the middle of the pitch. It's hard to break down when you get into their third, because they've got bodies compacted into the central area of the pitch. Jose Mourinho played one up and five in the middle of the park - that also made Chelsea very difficult to beat, but in a different way to what Ancelotti is doing. Ancelotti's got the two strikers, and that is a major change compared to Scolari and Ranieri."