Chelsea's 1-0 win, sealed by John Terry's late header, allowed Ancelotti's men to open a five-point lead at the top and left his United counterpart, Sir Alex Ferguson, once again aggrieved over his side's treatment at the hands of a referee.
While Ferguson admitted his side now face a tough task in attempting to close the gap on a Chelsea squad that is becoming as rich in confidence as it is in experience, Ancelotti was quick to underline the unexpected vibrancy of Arsenal's challenge as a reminder of the dangers of complacency.
"If Arsenal win their game in hand, they will be within two points of us," he said when asked to speculate on whether the title race was now a two-horse affair.
"I think also Arsenal is doing very well.
"A five-point lead is important, but at the same time, not so important. We have 30 points in 12 matches but we know the season is very long. We have to stay focused."
Chelsea fans' optimism that Ancelotti could emulate Jose Mourinho in wresting the Premier League trophy away from Old Trafford will have been encouraged by the fact that, three months into his first season in England, the Italian has now master-minded wins over both United and Liverpool.
"That is important," Ancelotti said. "But we lost against Wigan and Aston Villa. This means every game is the same. It is always three points and we have to maintain concentration against Manchester United, against all teams.
"It was very difficult to beat Manchester, the match was very balanced for 90 minutes and it was difficult for both teams to create chances to score. Sometimes we lost with the set plays, this time we won."
The set play that decided Sunday's match was a 76th-minute freekick awarded to Chelsea after referee Martin Atkinson -- mistakenly -- judged that Darren Fletcher, who had been outstanding for United, had fouled Ashley Cole close to the left touchline.
Frank Lampard's inswinging delivery was glanced into the net by Terry, although Nicolas Anelka also celebrated as if he had got the final touch.
Either way, Chelsea were fortunate to take all three points after a contest that a depleted United side had controlled for long periods, which only contributed to Ferguson's frustration over the critical free-kick.
"Clearly, Darren Fletcher's won the ball, Ashley Cole's never touched and has jumped up in the air, and then (Didier) Drogba's pulled Brown to the ground for the goal," Ferguson said.
"The referee's position to make the decision was absolutely ridiculous, he can't see anything. He's got a Chelsea player (Joe Cole) standing right in front of him and he doesn't even move.
"That goal shoudn't have been allowed. It was a bad decision, but there's nothing we can do about it. You lose faith in refereeing sometimes, that's the way the players are talking in there. It was a bad one."
Ferguson maintained sufficient composure to acknowledge that United could only blame themselves for failing, over the course of the 90 minutes, to turn their domination of possession into better chances.
"We had great opportunities to get to the edge of the box and some really good chances in and around the box, but we should be finishing it off," the Scot recognised.
"We've only ourselves to blame in that respect. But you do need a break - and we never got the break we needed."
Ferguson also admitted that the setback at Stamford Bridge was far more significant than last month's 2-0 defeat at Liverpool.
"The fact is Chelsea go five points clear of us," he said. "The defeat to Liverpool didn't affect our position in the league, this affects our position in the sense that if we'd have won we'd have gone top."