Lens coach Franck Haise admits they paid for their mistakes in defeat at Arsenal.
The Gunners won their Champions League encounter 6-0 in London.
Haise said: That's not what we were hoping for. I haven't seen all the goals yet, but I think we'll be disappointed with ourselves for at least the first four. When you make too many mistakes against a team like this, you pay.
We didn't have quite enough to repeat what we achieved at the Stade Bollaert-Delelis. We have to accept that, we have to recognise that. That's part of football. Obviously, it's a little hard to swallow.
"We can see that we're growing with every match, in both the good and bad moments. If we can manage to create some more memories for a that hasn't been in Europe for many years, with many inexperienced players, and an inexperienced club in general at this level, then I hope we can (qualify for the Europa League)."