United shipped three goals at Reading last week.
"It is a worry," said Ferguson.
"If we perform like that on Sunday then God knows what's going to happen to us.
"We've lost 32 goals already - I can't remember us losing so many goals before Christmas.
"Saturday was like Cartoon Cavalcade and when you see City have Balotelli, Dzeko - really tall players - it's going to be a big challenge.
"It's a concern. But that's on Sunday. It's five days away but at this moment in time it feels like five months."