The German defender dithered on the ball and Jesse Lingard's pressure proved effective. The ball was picked up by Romelu Lukaku, who fed it through to Martial in the box.
The French forward took one touch before delightfully flicking it on for Lingard arriving from behind. The England international steered his effort in off the post.
To further Arsenal's woes, Mustafi limped off the pitch having picked up an injury and was eventually subbed off.
Gooners forced to a early substitution due to the brutality of Lukaku, Martial n Lingard! Take it easy guys.. They have families.. @ManUtd #MUFC #ARSMAN
— Soheb (@SohebSM) 2 December 2017
Ars 0 - 2 Utd pic.twitter.com/9QIr372G4A