Ancelotti's side are 15 points behind United and, although they can reduce the gap with wins in their two fixtures against Sir Alex Ferguson's team, the Chelsea boss concedes even that would leave them with too much ground to make up.
Instead of reigniting a title challenge, Ancelotti knows Chelsea desperately need a win to boost their bid to finish in the top four and three points would lift the champions back above fourth-placed Tottenham.
"It is very difficult for us to close this gap. At this moment we have to think game by game," Ancelotti said.
"Man United can decide their destiny. Our aim is different in this moment. We want to arrive in the top four places, this is our aim. We have to believe but also be honest and objective."
Chelsea's hopes of a morale-boosting win would have been helped if United striker Wayne Rooney had been suspended for his elbow on Wigan midfielder James McCarthy in Saturday's match at the DW Stadium.
Referee Mark Clattenburg opted not to dismiss Rooney for the incident but the England star could have been hit with a three-match suspension - ruling him out of Tuesday's clash - if the Football Association had decided to take action.
However, Rooney has been cleared to play because the FA didn't want to over-rule Clattenburg's decision and Ancelotti refused to criticise the move.
"If the FA decide this I am happy to play against Rooney. I think this is right if that is the rule," Ancelotti said.
While Chelsea have faded badly after a superb start to the season, United have been able to pull clear at the top by grinding out results even when they haven't hit top gear.
Ferguson's men will go seven points clear of Arsenal if they win at the Bridge for the first time since 2002 and Ancelotti admits they have been the most consistent team this season.
"They play with consistency. They lost just one game against Wolves in the Premier League. Every time they are focused and concentrated, that's why they are top of the table," he said.
"It is a very important game for the future of United and Chelsea. We are excited to play this game.
"It is important to win to improve our confidence after the win in Copenhagen.
"Obviously it won't be easy. Until now Man United are the best team in the Premier League."
Ancelotti is hoping Fernando Torres can finally break his goal-scoring duck for the Blues against a United defence he often terrorised during his time at Liverpool.
Torres scored three times in six games against United defender Nemanja Vidic and tormented the Serbian centre-back into several red cards.
Ancelotti is likely to start with Torres alongside Nicolas Anelka, meaning Didier Drogba could be consigned to the bench once again.
"Fernando will be involved in the game and Didier will be involved, I don't know how many minutes they will play," Ancelotti said.
"They can play together without problem. We are trying to improve their combinations.
"For a striker to score is very important. In the last game Torres played well and had four opportunities. He moved very well for the team and I was very happy."