The game which takes place on Sunday at the Etihad Stadium is a chance for Man United to gain a six point advantage over their nearest rivals City.
This could prove pivotal lead into the Christmas schedule and Sir Alex was keen to stress how vital it was that his team improved their performances levels.
United lost 1-0 in their last fixture at the Etihad in what was then a crucial title decider to a Vincent Kompany header and Fergie insists he wants no similar lapses of concentration this time round.
Even though it is still early on in the season Ferguson insists that it would be a huge boost to get a result on Sunday and if they are to achieve this they need to shore up their defence.
He spoke to BBC sport about the 32 goals let in by United in all competitions this season, and Ferguson conceded: “It’s a worry.
He went on to add “If we perform like that on Sunday then God knows what’s going to happen to us.”
Martin Atkinson has been placed in charge of this Sunday’s game and Ferguson may be slightly more wary of his post-match comments about this referee after the match.
Fergie was handed an initial five match day touchline ban and a £30,000 fine by the FA last year following his comments about Atkinson when his side lost 2-1 at Stamford Bridge to Chelsea last season.