The two teams meet at the Westfalenstadion tonight both having lost their opening Group F matches, with Arsenal prevailing in France and the German outfit losing to Napoli.
"We knew it would be a tough group as soon as the draw was made," Mandanda told reporters.
"After the loss to Arsenal, it's even tougher. We really need to get a result in order to play an important role in this group."
Marseille make the visit to Dortmund on Tuesday.