The teams clash at the Etihad Stadium on Sunday (1330 GMT) 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." (Writing by Tom Pilcher, editing by Justin Palmer)
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