Per Mertesacker admits that Arsenal lacked “discipline and concentration” in their Champions League defeat to Olympiacos.
Arsene Wenger’s side are staring an early exit after losing their opening two Group F matches – away at Dinamo Zagreb and at home to the Greek side on Tuesday.
Arsenal now face two games against Bayern Munich, with Wenger admitting the Gunners must get a result against the free-scoring Bundesliga champions.
Mertesacker came off the bench in the second half of Arsenal’s latest defeat and the Germany defender admitted their performance was “not acceptable”.
He told Arsenal Player: “We lacked that concentration and it is not understandable why we drifted and did not do the job we are used to doing.
“That is not acceptable and we will be punished in the Champions League.
“We need to get behind the ball and play better defensively. We lacked discipline, even when you get a goal back don’t rush yourself.
“Get your discipline back and with possession you get your chances. If we are not disciplined and do not chase the ball we have got no chance.”
Arsenal go to Premier League leaders Manchester United on Sunday and Mertesacker is looking for a response from his team-mates.
“We have got a couple of days to look back on that performance,” he added.
“I think everyone will have regrets and it is good to take that away.
“It will take a couple of days to recover and to think about Manchester United and play in a different competition, which will be a good opportunity to come back to the Emirates and play our football with possession and dynamism.”