Last month's 2-0 defeat at Liverpool was a sobering experience for Alex Ferguson and was concrete proof the loss of Cristiano Ronaldo to Real Madrid and Carlos Tevez to Manchester City has severely diminished his team as an attacking force.
There are also worries at the back with Rio Ferdinand clearly off the pace as he struggles with a back injury and Nemanja Vidic, who was sent off against Liverpool, jittery.
While defeat at Stamford Bridge would not be catastrophic, Ferguson knows that a repeat of the Anfield debacle at Chelsea could put Carlo Ancelotti's team in the driving seat in the title race as winter looms.
"It's always a hard game at Chelsea, but we'll deal with it better than we did at Anfield, I'm sure of that," Ferguson told Inside United magazine on Wednesday.
"They're shaping up to be the main challengers. It seems they'll be the ones to get in front of."
Ancelotti's favoured midfield diamond is beginning to shine, giving the Blues both security at the back and potency up front where Didier Drogba looks back to his best after two goals in the 2-2 draw at Atletico Madrid on his return from a UEFA ban.
"Their consistency, compared to last season, has probably been a bit better under Ancelotti," Ferguson said.
"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.
"Ancelotti's got the two strikers, and that is a major change compared to Scolari and Ranieri."
"It's really important for us to beat United at home," Drogba said. "The match against Atletico was far from a good performance - I think we were thinking about Sunday's game."
