It was the most telling of Premier League showdowns—one that encapsulated the recent histories of both teams, and at the same time provided a snapshot of a rivalry fading in relevance.
At least it wasn't 8-2.
But in some ways, this defeat hurt Arsenal more than the one inflicted by United at Old Trafford at the start of the season. That one was a freak of a nature, a day where everything went wrong and everything went in.
This time, Arsenal met a more familiar demise—one their fans have seen all too often against lofty opposition. Arsene Wenger's team was outplayed and outfought at The Emirates. They were found wanting when their fans wanted it most.
"Spend some f***ing money," they sang, as Wenger's team drifted toward a third straight Premier League defeat. And as the minutes ticked by, so the mood of disharmony began to escalate into one of spirited mutiny.
It was Wenger's curious decision to replace the rampaging Alex Oxlade-Chamberlain with Andrei Arshavin that sparked the dissenters into life.
The bold young winger had no sooner laid on Robin van Persie's equalizer than he was replaced by the Gooner collective's scapegoat-in-residence. Even van Persie let on he was miffed. The crowd was far less subtle about it.
No sooner had Arsenal fans glimpsed a brighter future than their ineffectual present came back to haunt them. Wenger's substitution felt like a statement of lack of intent, and the revolution was being televised.
The elfish Russian entered the field to boos, and with his script waiting to be written. When he gifted Antonio Valencia the freedom of the penalty area, Danny Welbeck's emphatic winner for United damaged not just Arshavin's already-ravaged reputation but also that of the manager who paid a club-record £15 million for him.
"You don't know what you doing," they sang of the manager some still call Le Professeur. And as they filed angrily into pubs around the stadium, those who once posed as the Frenchman's disciples gathered around TV screens to spit bile in his direction as he took to the stand to defend himself.
"Oxlade-Chamberlain had started to fatigue, started to stretch his calf, and was not used to the intensity. He was sick during the week," said a defiant Wenger afterwards.
"Arshavin is captain of the Russia national team. I have to justify [substituting] a guy of 18 who's playing his second or third game? Let's be serious. I have to stand up for the substitutions I made.
"I've been 30 years in this job and have made 50,000 substitutions and I have to justify every time I make a decision? I do not have to explain to you every single decision I make."
For Wenger, the notion his tactics were flawed will surely have hurt far more than the continued criticism of his transfer policy. This is a manager with great pride in his achievements and reputation at Arsenal—one who has survived six trophy-less years on the faith he built up before them.
"In Arsene we trust"—so goes the famous slogan, but you won't hear it much this week. And if Arsenal fails to qualify for the Champions League this season and ends up losing van Persie to the highest bidder, you get the distinct sense they'll be calling loudly for Wenger's head come May.
Some are calling for it already.
There's only so much that football supporters can take, and when you consider the attitudes to managers at other clubs, Arsenal fans have been more patient with Wenger than most.
Now that they're turning on him, he faces perhaps the biggest test of character of his tenure at the club—fighting on at a club that's slowly falling out of love with him. From here on in, it's top four or bust for Wenger, and he knows it.