The pair meet with City having established a two-point lead over the champions last weekend and Sunday's Old Trafford encounter is one of the most keenly anticipated in years.
"The game could be significant at the end of the season, it is really a six-pointer, there is no doubt about that," Ferguson told a news conference.
"Whichever team wins it makes a difference. But it's early doors and you can recover from that, you hope with our record in the second half of seasons we will be better."
City are top with 22 points from eight games, having scored 27 goals, while second-placed United have also been finding the net with ease with 25 goals.
That does not been this derby will be a high-scoring one, with memories of last season's 0-0 damp squib at Eastlands fresh in many people's minds.
"It could be disappointing that way, it's a major game," Ferguson said. "It's very seldom in major games that you get a lot of goals."
The challenge posed by City is a similar one to the one United have faced from Chelsea - a quality team quickly built after a massive cash injection from a rich owner - and United are ready to deal with it.
"It's the type of challenge that we have accepted well over the years and you don't always want to be behind clubs, like we were for a couple of years with Chelsea," Ferguson said.
"But we showed with the vigour and determination and decision-making that we got back in front of that particular football club.
"On Sunday we have another situation where an emerging team like City have gone to the top of the league, early doors of course, there are a lot of winning posts along the way. One of these winning posts is Sunday."
Ferguson has often said United's fixtures with Liverpool, the club whose record they broke when they won a 19th English league title last season, are the biggest games of the season but he was not playing down the Manchester derby.
"It's any exciting prospect on Sunday," he said.
"I think City have done fantastically well, if they hadn't thrown away a two-goal lead against Fulham they would have been top of the league with a 100 percent record, It will be quite a game on Sunday."
The two clubs have already met this season in the curtain-raising Community Shield where United came back from two goals down to win 3-2.
Manchester City's last win at Old Trafford was in February 2008 when they won 2-1 and they will be hoping for a repeat even if United have not lost at home in the league since April 2010.
