Mikel Arteta says Arsenal will analyse what went wrong against Anderlecht because "it can't happen again".
The Gunners let slip a 3-0 lead at Emirates Stadium and a frustrating draw leaves them still needing a point to qualify for the last 16 of the Champions League.
"When you are 3-0 up, in a big game like this, an important game like this, because we could have been through the group stage, we missed a tremendous opportunity," Arteta told Arsenal Player.
"You can't afford to make mistakes like this and give any hope to any team in the Champions League because they have the quality to hurt you and that's what happened.
"You can't afford to make mistakes like this and give any hope to any team in the Champions League"
"We can't give those chances away because you don't know what's going to happen in the next [game]. When you have opportunities like this, being 3-0 up, it doesn't get any better than that, so we are very frustrated, very disappointed. We need to analyse what we've done because it's not good enough.
"We are too upset [to say anything else], but we have to analyse what happened and it can't happen again."
Arsenal's next match is at Swansea City - and Arteta sees Sunday's game as a chance to respond.
"That's the best thing in football, we have another game in four or five days against Swansea away, and it needs to be different," he said.
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