Mikel Arteta was at a loss to explain how Arsenal were beaten by Manchester United after playing their "best game of the season".
The Gunners controlled Saturday's big match at Emirates Stadium but squandered chances before falling behind to an unlucky own goal from Kieran Gibbs and a late second from Wayne Rooney.
Olivier Giroud's stoppage-time strike was not enough to rescue the points that Arsenal deserved and Arteta's disbelief was clear after the final whistle.
"We were controlling the game and hammering them. Afterwards you have to take more risks, to chase the game"
"I am shocked we did not win this game, I cannot believe it still," he said.
"I think we played our best game of the season, in terms of what we demand from ourselves with the quality of our attack, how early we won the ball back, how consistent we were throughout the game, the number of chances we created, and all that considering the opposition we were playing against.
"I can't believe it and think it is a shame, not fair for the players and the fans. It is very frustrating.
"We were controlling the game and hammering them. Afterwards you have to take more risks, to chase the game. I feel so sorry for the lads and for the fans as well, because they were terrific and we don't deserve it."
Arteta, who set up Giroud's goal, admits that Arsenal lacked ruthlessness when they were on top - especially in a one-sided first half.
"When you are inside the box, and you cannot manage to score in that many situations, you cannot expect to win a football game," he said. "On another day, we would have won the game very comfortably.
"We cannot forget [what we have done well] because it creates a lack of belief and confidence if you take a lot of negatives. Everybody is disappointed, but you can take many many positives from our performance. I was very proud with what we have done and I am shocked."