The teams clash at the Etihad Stadium on Sunday (1700 IST) with United three points clear of champions City. They have already opened up a gap on third-placed Chelsea [ Images ], who are a further seven points adrift.
"I think the two sides have that gap over Chelsea, Arsenal [ Images ] 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."
Meanwhile, Manchester City manager Roberto Mancini [ Images ] says his team simply cannot afford to lose Sunday's derby.
"They can lose. They have three points more. We have three points less and we can't lose," the Italian told reporters on Friday.
Champions City, unbeaten in 15 Premier League matches this season, won both encounters with their neighbours last season en route to a first league title in 44 years, including a 6-1 defeat of United in the first clash.
However, Mancini said United are always favourites because of their pedigree over the past two decades.
"We can't change the history. We are here (for) only two years, playing together, and they are used to playing this game for the title race for 20 years.
"This is normal when you play against a team that won everything in the last 15 years. You can't change this in two or three years and you need more time. For this reason, they have a small advantage.
"In this moment United are favourites but the season is long."
Mancini highlighted United's purchase of Arsenal striker Robin van Persie [ Images ] as a big momentum shift before the current season got underway.
"They were a strong team and then they bought Van Persie and (Shinji) Kagawa. They put another player that scores 25 goals into the squad," he said.