Per Mertesacker admits that Arsenal’s performance against Olympiacos on Tuesday night was “not acceptable”.

The Gunners face an uphill battle to qualify for the knockout stage of the Champions League after their 3-2 defeat at Emirates Stadium.

Mertesacker says the team fell below the required standard and must respond quickly with Manchester United due in north London on Sunday.

"We lacked that concentration and it is not understandable why we drifted and did not do the job we are used to doing" Per Mertesacker

“First of all, we are disappointed because we started quite well, especially on the break after they had set-pieces,” 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 [for that] 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.

“We have got a couple of days to look back on that performance," added Mertesacker. "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.”