There's a lot to get annoyed about with that performance. But if we respond well in the coming games, I think one messy, self-destructive loss can be accepted.
I'd say the fact that we have so few fit defenders caught up with us. Starting with two just coming back from injury in a makeshift formation and a midfielder and full-back playing at centre-back isn't ideal. Compounding that with two right-backs going off injured and ending the match with Monreal-Xhaka-Koscielny-AMN as your back four tells you everything really.
Even then, a bit more concentration at the back and more care up front could still have resulted in a positive outcome.
Leno has to take the blame for the winner. If we could transplant Cech's upper body onto Leno's legs, we'd have the perfect keeper. I'd stick with Leno at the moment, but any more unnecessary stuff like that and it will be a worrying trend rather than the odd brainfart.
I'd say only Guendouzi played consistently well today, despite Mkhitaryan's two goals.
Aubameyang missing very presentable chances is becoming costly now. As unconvincing as we were, there were still more than enough chances to get a win. Southampton's goal came after we had been on top for most of the second half, which was very much reminiscent of the pathetic capitulations that almost defined Wenger's latter decade at the club.
Most annoyingly of all, Austin will probably not score again until the next time we play at their stadium, Ings won't ever score another brace in his career and Long will play about five minutes of football for the rest of the season. If they had signed Chamakh and Heskey before the game, they would both have certainly pubbed a few in against us. St Mary's is worse than fucking Mordor.
Hopefully Bellerin, Kolasinac, Mustafi and Lichtsteiner are fit for the coming games because otherwise we're relying on Sokratis, Monreal and a patchwork Koscielny to get through the Christmas period.