Borussia Dortmund have been drawn in Group F with the Gunners, Napoli and Marseille, and the Champions League finalists' goalkeeper anticipates advancing could prove a challenge
The Gunners have been drawn in Group F alongside last year's beaten finalists, Serie A outfit Napoli and French side Marseille.
And Weidenfeller believes the north Londoners' deadline day capture of the Germany international could make progression to the knockout stages extremely difficult.
"Arsenal have gained enormous quality with Ozil," he told Bild. "Napoli have also reinforced. This will be a great challenge.
Dortmund have started the new Bundesliga season brightly, winning their first four games, but Weidenfeller acknowledges there is still plenty of work to be done.
He added: "We have come out of the blocks very well, with scores to be proud of. Nevertheless, we still have a lot of room for improvement."