Sir Alex Ferguson admitted Arsenal were worthy winners as United dropped their first points of the season on Sunday.
Emmanuel Adebayor's 85th-minute first strike gave Arsenal the points, following a surprisingly lethargic performance from the Reds.
"I can't deny Arsenal deserved to win the match," Sir Alex told Sky Sports. "We had some chances, particularly in the first half we had some excellent opportunities but not enough to win the match.
"I don't think we lost overall control of the match, but in the last 25 minutes I thought Arsenal were much stronger."
The Boss put his side's sluggishness down to the exertions of Wednesday night's gripping Champions League win over Celtic.
He said: "With the emotions of it and the pace of the match, it was a real Scotland-England type of match with that type of tempo. It was end to end all night and that's without question one of the things I have to look at.
"We lost momentum in the second half when we stopped passing forward, and that was a sign of tiredness. It was the second-half that disappointed me. We tired as a team in the last 20 minutes.
"It was a disappointing result for us. I don't think it was a bad performance but I think we can play better and at the end of the day Arsenal deserved to win."
Sir Alex cast a protective arm around Cristiano Ronaldo, who surrendered possession in the build-up to Adebayor's winner.
"I think he lost possession because he's always trying to beat men," he said. "That's the kind of player he is. That's the kind of player we want here. He's an exciting player and he wants to make things happen.
"Unfortunately he lost the ball and the midfield got caught out. These things happen in football. It could have been any one of the players who lost the ball."
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