The German snapped a selfie in the Gunners' changing room, with team-mate Danny Welbeck smiling prominently in the photo.
Welbeck, a former United player, scored the second goal of the match to help secure the win for the Londoners, with Granit Xhaka having grabbed the opener.
The result snapped a 25-match unbeaten run for United in the Premier League, and gives Arsenal renewed hope of a top-four finish.
Wenger Out Protest - Fans clash
Arsenal fan persuades #WengerOut protest THEY ARE WRONG "You're just wrong blud"Posted by Goal.com on Sunday, 7 May 2017