The Armenia international swapped Manchester for north London during the January transfer window, with Alexis Sanchez joining Jose Mourinho's side in a straight swap deal.
Mkhitaryan scored just once for United in the Premier League this term before agreeing his winter transfer to Arsenal, but was on the scoresheet against his former club during the Gunners' trip to Old Trafford on Sunday.
The 29-year-old was the recipient of a Granit Xhaka pass following a mix-up in the United midfield, before beating David De Gea from outside the area to level the game.
United had initially taken the lead through Paul Pogba, the midfielder scoring his sixth league goal of the campaign.
Mkhitaryan has now been involved in as many goals in his eight top-flight appearances for Arsenal this season as he had in 15 for United (six).