Goals from Klaas-Jan Huntelaar and Ibrahim Afellay condemned the Gunners to their first ever Champions League defeat at the Emirates Stadium at the hands of a foreign team.
And Bould expressed his belief that a shock loss to Norwich on Saturday had dented his side's belief and allowed Huub Stevens' men to leapfrog them at the top of Group B.
"I think the result on Saturday knocked us and we looked like we were lacking a bit of confidence," Bould told reporters.
"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."
The north London side face winless QPR in the Premier League on Saturday and Bould knows his side will have to pick themselves up if they are to turn their fortunes around.
"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."