Despite the final score, Arsenal started well and broke the deadlock after half an hour. Bukayo Saka cut the ball back for Martin Odegaard to deliver a low cross towards Jurrien Timber. The right-back's positioning may have flummoxed United defender Lisandro Martinez, who inadvertently deflected the ball backwards past Senne Lammens.
That seemed to spark a limp United into life, with Bruno Fernandes twice firing wide, once on the break and then when William Saliba played a stray pass, in the minutes after the opener. The hosts' uncharacteristically sloppy defending eventually cost them when Martin Zubimendi's loose pass was pounced upon by Bryan Mbeumo, who rounded ex-Brentford team-mate David Raya to equalise before the break.
On the other side of half-time, United pulled ahead in spectacular fashion. Fernandes flicked a bouncing ball on for Patrick Dorgu, who had worked his way infield from wide left, and he lashed a fierce half-volley off the underside of the crossbar and in.
Arteta made four substitutions on 58 minutes and completely emptied his bench by the final quarter of an hour to try and turn the tide back in Arsenal's favour. Once more, it was a set piece that helped them out. Lammens flapped at a corner and Mikel Merino was on hand to poke in at close range with six minutes to go.
But there was still time for one more twist. A flowing United attack saw Matheus Cunha drift inward and hit a driven effort towards the bottom corner, with Raya again powerless to keep it out as the visitors ran away with all three points.
GOAL rates Arsenal's players from the Emirates...
