Chelsea, two points ahead of the champions at the top of the table, have scored 19 goals and conceded only two in their last five games, both of those by Sergio Aguero - a possible Chelsea transfer target - in Wednesday's 2-2 Champions League draw at Atletico Madrid.
The club received a boost on Friday with news that they had been cleared to sign new players in the January transfer window after the Court of Arbitration for Sport (CAS) suspended a ban imposed by world body FIFA who found them guilty of inducing young Lens striker Gael Kakuta to break his contract.
But Ancelotti hinted after Friday's training that the club would only spend if his squad suffered significant injury.
After trotting out the usual cliches that the Manchester United match involved only three points like any other, Ancelotti said: "We are happy to play this game at this moment because we are playing very well, we are in good condition and we have a very good mental attitude just now."
Chelsea have only one player injured, defender Jose Bosingwa. Ancelotti said the decision about whether the Portugal back would need an operation on a knee injury would be made in the next few days.
The Italian coach, who has experience of both winning and losing against Ferguson's United while at AC Milan, said United could also be confident: "United have a very good team. Like us they are doing well because they have qualified in the Champions League and are doing well in the Premiership.
"But everybody is vulnerable and there are no invincible teams," he added. United have not won at Stamford Bridge since 2002.
Ancelotti said now was not the time to be thinking about signing players despite the CAS appeal decision.
"We have this possibility now but we have time. We don't want to make a decision at this moment. I am very happy about this squad, these players," he said.
Chelsea will lose several leading players, including in-form striker Didier Drogba and midfielders Michael Essien and John Obi Mikel for several weeks in January because of the African Nations Cup.
"But if we maintain players all fit I think we can manage a good period also without the African players," Ancelotti said.
Chelsea's scouts have been busy in recent weeks, however, and the club has been linked strongly in the British media with Aguero.
The Italian added that United were not Chelsea's only realistic rivals for the title. Liverpool, he suggested, were probably out of the race but Arsenal looked dangerous, he said.
