Arsenal boss Arsene Wenger was left disappointed with their defending after defeat to Champions League opponents Borussia Dortmund.
After Oliver Giroud had cancelled out BVB's opener from Henrikh Mkhitaryan, Robert Lewandowski netted struck the winner.
"We put ourselves on the back foot with the first goal and naive for the second goal," he said. "If you look at the number of saves our keeper had to make tonight and you concede two goals, you can say that we can only look at ourselves for not being mature enough in situations like that.
"If you cannot win the game, you don't lose it. Especially in the second half when we were on top, we made things difficult for ourselves by giving them the second goal."
