Steve Bould admitted Arsenal are suffering from a lack of confidence after they were beaten 2-0 at home by German side Schalke.
The Gunners showed a worrying lack of penetration as they failed to convert their midfield possession into chances.
And they paid the price late on as Schalke exposed some defensive frailties to score twice and clinch a 2-0 win - a win that sees the Germans leapfrog Arsenal at the top of Group B.
Bould thinks the side has struggled to recover from their weekend defeat at Norwich.
He said: "I think the result on Saturday knocked us and we looked like we were lacking a bit of confidence.
"We haven't looked at our best since we came back from the international break, we've looked a bit tired and jaded, but that shouldn't be an excuse."
And the assistant manager admitted that the side are showing worrying signs at both ends of the pitch.
"We certainly don't want to be conceding goals like that and if we'd have escaped with a 0-0 tonight we'd have been delighted," he continued.
"We're not making many chances either which is a concern.
"We look like we're lacking confidence on the ball which is unusual for us and we need to correct it quickly because we've got a big game on Saturday against QPR."