I didn't expect them to drop points off against Lens though, they shouldn't have lost that game. They should be very focused when they face Sevilla ahead. Sevilla now has a new coach, so they should expect something different and tough.
Arsenal was a better team than Lens, but I'm not sure what happened to cause them to lose to Lens. Their forthcoming game against Sevilla will not be as easy for them to win because Sevilla is a stronger team than Lens as well, and Arsenal will need to put in more effort in this match to win. Sevilla has been quite good in European competitions, as evidenced by their prior record in the competition over the years. The game will be played at Sevilla's stadium, and their head-to-head record is even.
This is a surprise that sometimes cannot be predicted because after all the favorite does not mean they can win easily as well as Arsenal in the match against Lens.
They excel in everything in terms of attack and their performance is better, it's just that matches like this and Arsenal's defeat are difficult to predict even though this is not impossible for things like this to happen.
To fight Sevilla, the two clubs are currently still at risk because Sevilla sometimes surprises when playing against clubs from the EPL but on the other hand there is another tough match for Sevilla because in domestic competition they will also face Madrid which of course this will also make them tired.
Arsenal are still favorites in this match but of course the same incident as the defeat against Marseille can also still happen.