"It is a little bit of an injustice," he told ZDF. "But we had that bit of luck in London [where we won 2-1] and today we were punished.
"We settled into the match immediately and had great opportunities. But we lacked determination in the decisive moments. We were not energetic enough and always tried to play nicely and maybe do another pass and another pass after that.
"We gave away big points. Group F is totally open now. We have to win against Napoli and best thing would be to win by two goals."
Dortmund are currently third in the group with six points from four matches, three points behind both Arsenal and Napoli.