The Red Devils travel to the Etihad Stadium on Sunday with a three point advantage in the Premier League table - although City have a superior goal difference.
United have already conceded 32 goals this season, in all competitions, including three at Reading last Saturday.
Skipper and central defender Nemanja Vidic is unlikely to return from injury and Fergie is certainly a worried manager heading into this important derby match.
He said: "If we perform like that (Reading match) on Sunday then God knows what's going to happen to us.
"We've lost 32 goals already and I can't remember us losing so many goals before Christmas.
"Saturday was like Cartoon Cavalcade and when you see City have Mario Balotelli and Edin Dzeko - really tall players - it's going to be a big challenge. It's a concern."