Aaron Ramsey believes Arsenal paid the price for switching off as they lost to a late Dortmund counter-attack.
The Gunners slumped to their first defeat since the opening day of the Premier League, and in-form Ramsey says Arsenal got what they deserved for switching off.
Dortmund's first goal came directly from the Welsh wizard surrendering possession on the edge of his own area.
Henrikh Mkhitaryan punished Ramsey's error, finishing past Wojciech Szczesny and giving the away side the lead.
And the midfielder accepts responsibility for Dortmund's first goal:
"I was disappointed as well with the way they scored their first goal," he told Arsenal.com. "I got caught on the ball and that was disappointing."
The Gunners struggled to play their usual free-flowing football, however, Ramsey feels his side worked their way into the game.
Arsene Wenger's side were enjoying a spell of dominance when Dortmund scored the winner on the break, and the result left Ramsey disappointed.
The former Cardiff player has been in prolific form this season, already finding the net nine times.
Ramsey believes the Gunners may have even grabbed a win in other circumstances:
"We showed good character getting back into game and we created a few chances in the second half and hit the bar. On another night we may have taken the lead but we have to learn from the way we defended for the second goal.
"A draw would have been a fair result after the way we played in the second half," he told Arsenal Player.
"I felt like we were in control of the game but then we switched off for 30 seconds, we were in their box and 30 seconds later the ball is in the back of our net.
"We were in control of the game, we didn't look like we were going to concede, but then we did. That's what happens [when you switch off]."
The result gives added to importance to their trip to Germany as the winner of the tie will be in pole position to win the group.
Arsenal will be looking to get back to winning ways when they take on Crystal Palace this weekend.