Everything can still happen to them and the worst possibility such as injury must be anticipated from the start because it is clear that currently there are several clubs who have experienced the severity of the injury storm which has made it difficult for their clubs such as Chelsea in wading into the EPL and of course Barcelona who must return to Europe. League due to injuries to their players.
Currently Arsenal are in very good condition so they have to anticipate all bad possibilities for them to continue to be consistent in the EPL and Europa League.
I believe Arsenal are one of the favorites in the Europa League this season. Their game is very good and I watch their game more often every chance I get, it's very entertaining. But you're right, player injuries are the most likely worry that every team thinks about right now given the very busy schedule.
But for now the round of 16 matches are still very far from Arsenal. They still have time to stay focused on the Premier League while waiting for who will be drawn for the next opponent. So it's still safe to say Arsenal are the favorites if they're only going to meet weak teams in the last 16.
Arsenal will be counted as the favourite considering their performance which is quite stable on premier league or europa league but their opponent hasn't determined yet so they have to wait the results of play off matches but Arteta has aware the possibilities Arsenal will meet the teams who eleminate from Champion league such as Barcelona or Juventus so probably in the round of 16 Arsenal will meet strong opponent and this makes Arsenal journey will be more hard to achieve the trophy but although Arteta didn't promised the trophy but he said Arsenal will attempts to get good achievements on europa league but indeed it's difficult to focus to several different competitions and probably Arsenal main target is premier league