Arsene Wenger admits he was left "angry and disappointed" after Manchester United won 3-1 at Arsenal on Saturday night.
Defensive mistakes by Laurent Koscielny and Shkodran Mustafi allowed Antonio Valencia and Jesse Lingard to hand United a 2-0 lead inside 11 minutes at the Emirates Stadium.
Arsenal refused to buckle, and though Alexandre Lacazette netted when United's defence stood still, David de Gea made a string of fine saves before Lingard struck again on the counter-attack.
Paul Pogba did see red for a dangerous tackle on Hector Bellerin, but Arsenal were made to pay for a poor start, leaving Wenger frustrated as their perfect home record this season came to an end.
"At the back we didn't start well at all," Wenger said. "We put ourselves in a very difficult position. Despite that, we had enough chances to come back.
"When you have 33 shots on goal in a game of that stature, it means our dominance was top class. Our spirit was outstanding, the quality of our performance was outstanding.
"But I'm angry and disappointed because at the end we have nothing to show for it. In games like that you have to be efficient."
Mustafi was caught napping by Lingard for United's second and was replaced immediately afterwards, limping off apparently with a right ankle problem.
Yet, tellingly, Wenger did not shed any further light, adding: "I don't know what's wrong with him at all."