Arsenal's performance against Lens was not bad. There was some strategic mistake so they lost. Arsenal's overall performance in this match was very good. I don't think Arsenal will make the same mistake again. Arsenal team is strong enough. Sevilla's performance, on the other hand, has been weak and erratic. I don't think Arsenal will make any mistakes against this team. If Arsenal's defense can perform a bit better in this match, I think they can beat Sevilla. Sevilla team can be considered as the weakest team in Group B.
Sevilla's performance is indeed erratic but that doesn't mean they are the weakest team in group B, I think Arsenal still have to be wary of the Sevilla team even though overall if we look at the strength of the two teams then Arsenal is much better than the Sevilla team.
The defeat against Lens will certainly be a lesson for the Arsenal team and they cannot repeat the same mistakes if they still want their chances of qualifying from group B to be wide open from what happened and the big motivation is definitely from Arsenal. I am also very sure they will be able to win over Sevilla but that doesn't mean it will be easy for Arsenal.