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."
United lacked fluency in their last Premier League game, a 2-0 home win against Blackburn Rovers, while against CSKA Moscow in the Champions League at Old Trafford on Tuesday their defence, minus Ferdinand and Vidic, was a shambles.
Two goals in the last five minutes inspired by substitute Wayne Rooney earned United a 3-3 draw to secure their place in the knockout round but it was a performance that offered plenty of encouragement to Chelsea.
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."
While United struggle to fire on all cylinders, Drogba said Chelsea have a great chance to put some early distance between themselves and their biggest rivals.
"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."
United could well be without Ferdinand against Chelsea while Gary Neville is suspended but Vidic and Ryan Giggs, both of whom missed the CSKA game are likely to return.
Fast-improving Arsenal can creep above United on Saturday if they win at Wolverhampton Wanderers.
Fourth-placed Manchester City take on Burnley hoping to end a sequence of four consecutive league draws while Liverpool, who conceded a late equaliser in their disappointing 1-1 Champions League draw at Olympique Lyon, try to revive their flagging season at home to Birmingham City on Monday.
