Mikel Arteta said Arsenal would need to “reset” after slipping to just a second defeat of the season in losing at PSV Eindhoven.
The Gunners needed just a point to secure top spot in their Europa League group but were beaten 2-0 in the Netherlands as Joey Veerman and Luuk de Jong scored the goals.
The hosts also had three goals disallowed for offside as Ruud van Nistelrooy’s men confirmed their own place in the knockout stages.
Arsenal sit top of the Premier League and had won their opening four Group A games in the Europa League, with Van Nistelrooy’s former club Manchester United the only side to beat them so far this season.
Speaking after the defeat, Arteta admitted his side did not reach the standard he demands from his players.
“I’m very disappointed,” he said. “It’s the end of a long run where we won a lot of games and we have to reset.
“Congratulations to PSV – they were the better team today. We were nowhere near our level and we know that in Europe it’s very difficult to win.
“We didn’t get any control. We didn’t get the ball in the areas we wanted. We gave so many balls away, which allowed so many transitions, which was what they were looking for whenever they got in the right structure.
“They were on top of a lot of things that last week was different.”
Like last week, when his side lost 1-0 at the Emirates Stadium, Van Nistelrooy felt PSV grew into the game.
“The first half wasn’t easy,” he said.
“I saw a number of the boys running out of energy because we had to chase the ball so much. The second half was much better.
“We pressurised them, we were much more dangerous and didn’t give much away.”
Arsenal host Zurich in their final group fixture and, should PSV win at Bodo/Glimt, only a victory for the Gunners – two points clear at the summit as things stand -would be enough to seal top spot.