An unbeaten run of nine games, stretching from a hard-fought draw in Donetsk to this remarkable result, has forced those who've been knocking David Moyes' club to rethink their stance.
The Gunners remain top of the league, but their lead has shrunk to two points as United has climbed to fifth after banking their third domestic victory on the bounce.
What made United's triumph even more impressive was the XI they used in the win. Shinji Kagawa and Chris Smalling played out of position, and Phil Jones split his time between defence and midfield.
We'll focus on Smalling today and outline what a great job he did from right-back.
Centre-backs at right-back
It's a common occurrence in football, as once a manager runs out of fit full-backs, he'll look in three directions for the answer: a holding midfielder, a traditional winger or a centre-back.
Martin O'Neill used both Carlos Cuellar (CB) and Nigel Reo-Coker (DM) at right-back during his time at Aston Villa when forced to change his XI, and Moyes has shown a propensity to start Smalling there while Rafael is out injured.
He initially looked awkward but has grown into the role and its requirements quite quickly, and he now represents a viable outlet on the flanks. He's not slow, his stride isn't loping, and he is, from a basic level upward, a conscientious and honest defender, aware of his responsibilities and limitations.
Converted centre-backs with good size will rarely behave the same as groomed full-backs, and Smalling is no different, as the England international plays noticeably narrower than Rafael.
He drifts toward his comfort zone, and, to balance that out, Antonio Valencia drops deep to track runners steaming up the outside.
Moving forward, Smalling is limited, and United do large amounts of their attacking through the marauding Patrice Evra and Shinji Kagawa. Smalling acts a stabiliser and can slot into a three-man defensive presence when Evra hits the byline at the other end of the pitch.
Positional play, instincts
Clever strikers are able to manipulate converted full-backs and create space around them.
Smalling's natural inclination to shift inside will be played upon, and late in the second half, Nicklas Bendtner was able to lead him away to create space for a clear-cut chance.
Serge Gnabry raced down the right and crossed a low ball into the box. Olivier Giroud was able to receive it, run it across his body and angle for a shot from 15 yards.
The reason? Bendtner had dragged Smalling out of the way, across the box and into a left-centre-back position. He was able to identify what had happened quickly, though, and get back across to block the shot and preserve the clean sheet.
Smalling follows his man, as he should, but breaks off his run at the right time to address the new threat. There's a strong chance Rafael doesn't register this, and if that's the case, we're looking at a different scoreline.
Wenger blows it
Aside from the final 10 minutes, Arsene Wenger matched up Santi Cazorla against Smalling from the left flank. The Spaniard also has an inclination to drift inside—he fails to hold the width on a regular basis—and this played into Smalling's hands.
A player such as Lukas Podolski, one able to hold the width, would have been a nightmare for Smalling, but the German remains injured and out of action.
The decision to put on Bendtner was good on paper: Arsenal need a goal, so send on another striker.
But a player looking sharper and fitter could have caused Smalling a few problems, and Wenger may have found joy in attacking the right-back in a similar fashion to how Mesut Ozil and Gnabry attacked the left.