Arsenal have been described as the team who “don’t want to defend” by Alfred Finnbogason, the Olympiakos striker, who scored his club’s winning goal in the Champions League match at Emirates Stadium on Tuesday night.

The Arsenal inquest has focused on Finnbogason’s 66th-minute goal, which came less than 60 seconds after Alexis Sánchez had equalised to make the score 2-2 and was the result of a collective loss of discipline that left the home side exposed.

Per Mertesacker, the Arsenal defender who came on after Laurent Koscielny had injured his hamstring, has said his team will not qualify for the competition’s knockout phase if they continue to show such failings.

After losing their opening game at Dinamo Zagreb, Arsenal may need to get something from their home and away ties against Bayern Munich. Koscielny is not expected to be fit for Sunday’s Premier League game at home to Manchester United.

Finnbogason, the Iceland international who was a half-time substitute, talked of how Olympiakos had executed a perfect gameplan after scrutinising Arsenal’s weaknesses, which include how they defend corners. Olympiakos’s first goal came from a corner which was drifted back to the edge of the area for Felipe Pardo to score with a deflected volley.

“The first corner is something we practise on the training pitch because we know they have no man on the edge of the box,” Finnbogason said. “When you play against this team you know they’re going to have a lot of the ball. You have to defend well, have the lines tight and use your time when you have spaces. And you know you’re going to have spaces because they don’t want to defend. So our plan worked.”

Arsène Wenger had substituted the defensive midfielder Francis Coquelin for the more attack-minded Aaron Ramsey on the hour and Mertesacker suggested there had been a rush of blood and a lack of balance after Sánchez’s equaliser.

“We rushed a bit and not everyone came back to a defensive position, like we should have done,” Mertesacker said. “I felt we were a bit exposed. There was no one on the edge of the box. We should have done much better, especially having come back and had a lot of possession in the second half.

“After we scored it looked like we were not ready to win this game. There was a lack of concentration, a lack of discipline. We need to do much better defensively on these occasions because these occasions decide more and more games in modern football. When we play like that, with that lack of concentration and discipline, we have got no chance [of qualifying].”

Gabriel Paulista, the other Arsenal centre-half, said it was essential the team regrouped before United’s visit. “It was a complicated night,” he said. “We only have ourselves to blame for conceding the three goals and now we have to rest, work harder and, on Sunday, we have a great game.

“Every game is very important and Sunday is the same. We are at home and we have an obligation to win at home. Things are not turning out well in the Champions League. In the Premier League we are fine but in the Champions League – I don’t know what to say.”