Ferguson: Title race between Manchester duo
The teams clash at the Etihad Stadium on Sunday with United three points clear of champions City. They have already opened up a gap on third-placed Chelsea who are a further seven points adrift.
"I think the two sides have that gap over Chelsea, Arsenal and Tottenham. It will be difficult to think the two of us will collapse. I think it will be between the two of us," Ferguson told reporters.
Even if United end City's unbeaten start to the season on Sunday, Ferguson said a six-point advantage at this stage of the season would not be that significant.
"It won't mean a great deal because there's such a long way to go until the end of the season," he said. "You can never be sure and you can lose to anyone. That's the kind of league we're in. It's a tough league."
United will be without wingers Nani, who has a hamstring injury, and Antonio Valencia, who has a hip problem, for the derby after Ferguson revealed the pair would be out of action for a "few weeks".
Captain Nemanja Vidic, who had been touted for a midweek return in the Champions League, and playmaker Shinji Kagawa will also miss out and are now set to return against Sunderland the following week.
"I thought Kagawa may have done enough to make the bench on Wednesday but no, he's well short," Ferguson said.
"But I don't think he'll be available until the Sunderland game. Vidic has certainly been doing well in training and it suits his own confidence to play but I think the game on Sunday is a bit soon for him. I think he'll be ready for Sunderland.
"I know we've got a few injuries at the moment but we've got a big strong squad of players. Nani and Valencia are still out. It will be a few weeks with them."