The win at the San Siro followed four successive defeats for the Gunners, including a 3-0 defeat to Manchester City in the Carabao Cup final.
It was the first goal Milan had conceded in 599 minutes and they should really have been punished further, with Danny Welbeck shooting straight at Donnarumma one-on-one and Mkhitaryan clipping the crossbar with the goal at his mercy.
Gattuso introduced Nikola Kalinic and Andre Silva to bolster Milan's limp attack, but speculative efforts from Bonaventura and Kessie were about the best the home side could muster, allowing Arsenal to head back to London with a solid lead and a major boost in confidence.
While Koscielny, Ramsey and Ozil all had strong cases to be named man of the match, Mkhitaryan shaded it after producing one of his best performances in an Arsenal shirt.
Cape Town - Arsenal defender Sead Kolasinac has urged his team-mates to pick themselves up and do their utmost to perform against AC Milan on Thursday. He wants his players to be competitive and aggressive: he was like that as a player, so I'm not surprised.
Arsene Wenger: Win at AC Milan to 'lift the belief' at Arsenal
Arsenal earned their first win in the opening leg of a last-16 European tie since February 2011 (2-1 v Barcelona in the Champions League).
Arsenal fans aren't going into their Europa League clash with AC Milan tonight with any optimism, it seems.
Conversely, the Gunners are coming off the back of four defeats - leaving their top four ambitions in tatters.
That assist saw him reach a notable landmark, with a hand having been played in 100 goals.
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