Alex and Ramires have failed to recover from hamstring injuries and will miss Chelsea's Barclays Premier League game against Birmingham.
The Brazilian duo were expected to be fit after sitting out Saturday's win at West Brom but will now be unavailable until after this weekend's clash with West Ham.
Chelsea are eight points behind United ahead of the leaders' trip to Newcastle on Tuesday.
A defeat or draw for Sir Alex Ferguson's men would give the Blues a slim chance of hauling themselves back into the title race but Ancelotti said: "The gap doesn't allow us to think we can win the title.
"So it's better for us to think about preparing for the next game against Birmingham to try to win, and the other five games."
Much more realistic is overhauling second-placed Arsenal, which Chelsea will do if they beat Birmingham and the Gunners fail to win the north London derby.
However, Ancelotti once again demonstrated the level of expectation at Stamford Bridge by saying: "Second or third place is not important for Chelsea."
November's defeat at Birmingham was one result which the Blues might look back on as ultimately costing them their title.
"That game was one of our best performances this season. We didn't win and were really unlucky," said Ancelotti.
"They were in the relegation positions, but had very good results in the last few weeks.
"They're not out of fighting from relegation entirely, so they need to take a point tomorrow. It will be a tough game.
"They have good organisation and will try and play on the counter-attack, as they played against us in their home game."
Birmingham winger Jean Beausejour is back in contention after injury.
The Chile international missed the 1-1 draw at Blackburn and the 2-0 home success over Sunderland at the weekend with a hamstring strain.
City boss Alex McLeish, whose side defeated Chelsea 1-0 at St Andrew's in November, has no fresh injury concerns, but strikers Nikola Zigic and Obafemi Martins plus defender Martin Jiranek remain sidelined with groin problems.
McLeish knows it will be a major achievement to emulate their win over Chelsea in November, courtesy of Lee Bowyer's goal, or even obtain a point.
But he has been pleased with the response of his players in picking up seven points from the last three games.
He said: "Chelsea looked happier at West Brom on Saturday with the one player up front.
"We know if we get anything at Chelsea, we are punching above our weight.
"We got a bit of luck that day we beat Chelsea at our place, some great rearguard action, great goalkeeping.
"But I'm just really pleased with the way the players have responded in the recent games.
"Results can go against you due to no fault of your own, a decision, a bit of bad luck, but you must continue to give the performances.
"We've actually played some good football. Maybe we don't build up the way Barcelona and Arsenal do but then again not many teams do.
"But when we get the ball down, we've got some players who are terrific on the ball and you've seen that."
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