Arsenal manager Arsene Wenger accused his side of being "naive" as they suffered their
first Champions League defeat of the season,
beaten at home by Borussia Dortmund.
Robert Lewandowski's late volley gave last season's beaten finalists a 2-1 victory at the Emirates.
Arsenal stay top of Group F level on six points with Dortmund and Napoli.
"We put ourselves on the back foot with the first goal and were naive for the second," Wenger said.
Gunners keeper Wojciech Szczesny had little to do in the match bar picking the ball out of the net,
Wenger said: "Look at how many saves our goalkeeper has had to make and we have conceded two goals.
Arsenal's remaining Group F fixtures
"We can only look at ourselves as not being mature enough in those situations. If you can't win the game, don't lose it."
Armenia striker Henrikh Mkhitaryan opened the scoring for the visitors early in the first half after Gunners midfielder Aaron Ramsey had been caught in possession.
Wenger's side equalised just before the break through Olivier Giroud, and had looked the better team in the second half before Lewandowski's sucker-punch eight minutes from time.
On Arsenal's chances of reaching the knockout phase, Wenger said: "Tight and difficult, but possible."
Match-winner Lewandowski was fortunate to be on the pitch to score the decisive goal after appearing to elbow Laurent Koscielny, with Wenger saying: "It looked like that. I honestly haven't seen it all but the referee was in a much better position than I was.
"It was a game of high physical intensity. Dortmund were very well-organised to stop us from playing. I must admit we were not as good as we have been and some of our players were not as fresh physically."
Jack Wilshere was taken off in the second half and immediately placed an ice pack on his right ankle. The midfielder had already received lengthy treatment in the first half.
Wenger said: "You could see that he was not completely himself and that is why I decided to take him off. It is his desire to play that gets him through a game.
"He looked like he landed awkwardly and after that, looked like he limped a little bit. We will see how he responds in the next two or three days."