The Poland international was disappointed after seeing Nuri Sahin and Sven Bender pick up injuries ahead of Wednesday's crucial Champions League trip to Marseille
Jurgen Klopp's side slipped 10 points behind Bayern Munich, with Nuri Sahin and Sven Bender picking up injuries to join the likes of Ilkay Gundogan, Marcel Schmelzer and Mats Hummels on the sidelines.
They have now lost three of their last four Bundesliga matches and the Polish striker believes their crisis is starting to get the better of them ahead of Wednesday's crucial Champions League clash with Olympique de Marseille.
"We did not play so well both in attack and in defence and that was the problem," Lewandowski is quoted as saying by Der Westen.
"Of course, it is a problem that many players are injured and now maybe two others are. This is a very difficult situation at the moment."
Meanwhile, goalkeeper Roman Weidenfeller admits that BVB need to get to the turn of the year without losing any more games in order to have any hope of turning their season around.
He added: "For us it is critical to get some points now so that we can go into the winter break quickly then start afresh in January."
Dortmund now face a must-win visit to the Stade Velodrome but club president Reinhard Rauball is certain that they will rise to the task.
"You have to make the best of it," he reasoned. "You can lose, you can fall, but we have to get up again and pool our resources. I can promise we will do that.
"And I do believe that we will put out a strong team on Wednesday."