The current league leaders have conceded 32 goals in all competitions this season, and Ferguson conceded: "It's a worry.
"If we perform like that on Sunday then God knows what's going to happen to us."
Speaking before Wednesday night's Champions League game against Romanian side CFR Cluj, Ferguson said: "When you see the goals against - we've lost 32 already - I can't remember us losing so many goals before Christmas.
"It's a concern, there's no question about that. When you see City have Balotelli, Dzeko - all really tall players - it's going to be a big challenge."