Arsenal boss Arsene Wenger admits his players got complacent after taking a 3-0 lead against Anderlecht on Tuesday night.
The Gunners looked set to take all three points from the Emirates but two goals from Anthony Vanden Borre and a late header from Aleksandar Mitrovic sees Arsenal trail Borussia Dortmund by five points in Group D.
And Wenger was yet again critical of poor defending from his players.
"You have to give them [Anderlecht] credit because the fought until the end and we had a poor defensive performance from the first to the last minute today," Wenger said after the game.
"We never felt comfortable and somewhere we got punished because of bad luck. The first goal was offside but defensively our performance was not good enough to do it.
"Secondly at 3-0 we thought the job was done and it was a combination of switching off and fatigue as well.
"You could see we couldn't win the challenges and they finished stronger and sharper - we got punished. It's very disappointing but overall over the two games we got four points.
"I think we are well paid for our two performances.
"I think when we lost Mikel Arteta we lost out because he was one of the few who defended well. That didn't help.
"Apart from that maybe we underestimated subconsciously Anderlecht and got punished. In the Champions League you need to be at the mental level or you get punished and certainly we were not."
Asked if he will strengthen the defensive department during January's transfer window, Wenger said: "Honestly it's not the worry tonight.
"We just came out of two clean sheets and maybe thought we had got the job done at 3-0. The target is to go to Swansea with another [good] defensive performance.
"I also believe the luck we have is that we still have a good chance of qualifying. If we had lost tonight, what could have happened, would have been a very bad situation for the qualification.
"For winning the group, tonight you can say that's nearly impossible now."