Louis van Gaal admits he was “amazed” at how Arsenal outfought his Manchester United side in the 3-0 defeat at the Emirates.
The Dutchman said: “I was surprised. Not performing to our game plan, not the will to win. I have not seen that when you are top of the league.
“You need aggression at the moment you have a duel and Arsenal were much more aggressive.”
Alexis Sanchez hit two and Mesut Ozil scored the other – all in the first 20 minutes – as Arsene Wenger’s men leapfrogged the Red Devils into second place.
Van Gaal added: “I was surprised, amazed - maybe that is a better word.
In pictures - Arsenal 3-0 Manchester United
"When you give a team like Arsenal so much space to play football then you know you will lose.
“There was a lack of ball-winning.
"They were not slow, but they were not aggressive.
"Players like [Michael] Carrick and [Bastian] Schweinsteiger have a fantastic positional sense. But you need aggression and winning the second ball. And you have to do it not only with the two in midfield but also as a team.”
Frenchman Wenger believes man-of-the-match Theo Walcott, who set up two of the goals, made a huge statement in his bid to become the club’s central striker.
He said: “I was always convinced. When a guy plays well it’s difficult to find people who are not convinced.”