"It's difficult to take because I would agree that we were maybe not at our most fluent tonight going forward, but at that moment we were on top," the Frenchman told Sky Sports News.
"We lost the game because we maybe wanted so much to win it that we forgot that if you cannot win it, don't lose it. That's what is most annoying because we put a lot of effort in.
"We started hesitant. At the end of the day, we went 1-0 down and when you don't start well it is difficult to get back into the game. But we did well and in the second half we were on top of it. We just forgot to be cautious. We were not cautious enough and we paid for it."
Arsenal, Dortmund and Napoli are all on six points from three matches in Group E.