#4 Europa League ineligibility
A problem that will only persist in this season, but it isn't a small one. Dortmund's failure to survive the UEFA Champions League "group of death", containing Real Madrid and Tottenham Hotspur, and their subsequent drop into the UEFA Europa League means that Aubameyang cannot play for Arsenal in the competition.
According to UEFA guidelines, a player is not allowed to play in the same competition as his former team is currently in. If Dortmund had qualified from their Champions League group, Aubameyang would have been allowed to play and the worst part about it, is that Arsenal have their North London rivals to thank for this predicament.
While the Europa League has been subject to plenty of debate in recent years and isn't classified as being very important, Arsenal may find their best chance of making it to next year's Champions League in Europe's secondary tournament.
The Gabon international will be restricted to Premier League football and the EFL Cup final for the remainder of the season. While Arsenal's top-four bid isn't over by any means, they would have obviously liked the option of fielding the striker in the competition.