"I think that I'm very happy, it was an important match and a very difficult match," the Italian, Chelsea's fifth manager in less than two years, told reporters after his side won 4-1 on spot kicks at the end of a 2-2 draw at Wembley.
"They had a very strong start to the game and we found it very difficult to play in the first half. But we played better after that and we deserved to win.
"It's an important start to the season because we have changed the way the team plays a bit. I'm very happy not only for this month but also for this past month.
"I now know the players, I like their mental attitude and I like the organisation of the club. We're ready to start a very important season for us and we are ready to do our best."
Ancelotti's attempt to wrestle the Premier League title away from United begins on Saturday when they host Hull City.
The Italian, who had great success as a player and coach with AC Milan, winning two Champions Leagues and a Serie A title during his eight years in charge at the San Siro, said beating United was always important, whatever the occasion.
"The match was balanced," he said. "Maybe they were a little bit better in the first half and Chelsea were a little bit better in the second half.
"Manchester are our most important opponents in the Premiership because they are a great team, they have great experience. So this gives us a bit more trust.
"But now the game is finished we have to think of the season and the next matches."
Ancelotti played down the controversy over Chelsea's second goal, scored by Frank Lampard when United defender Patrice Evra had been flattened by a crude bodycheck by Michael Ballack.
With referee Chris Foy waving play on, United's players expected Chelsea to kick the ball into touch but instead Didier Drogba surged towards goal before picking out Lampard to put Chelsea 2-1 ahead with 20 minutes remaining.
"The referee can stop the play and also the players can put the ball out," Ancelotti said. "But they didn't see Evra on the pitch so they continued . for sure if they had seen him on the pitch they would have put the ball out."
(Editing by Sonia Oxley)