For years, theirs was the sort of bitter rivalry that marked them out as two halves of one whole. It was the sort of hostility that could only come from similarity. While the arguments raged, the likenesses outnumbered the differences.
These were evenly matched enemies to the extent that determining the superior in each individual match-up — Arsene Wenger versus Sir Alex Ferguson, Patrick Vieira or Roy Keane, Peter Schmeichel against David Seaman, Marc Overmars or, later, Robert Pires instead of Ryan Giggs — was sufficiently difficult to provoke a dispute in its own right.
Now the relationship has evolved. Rather than threatening to be Manchester United's nemesis, Arsenal are their antithesis. With every setback, the unwanted comparisons mount.
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Concede late goals? United score them. Lose leads? United do not. Fail to convert chances? United take theirs. Lack mental strength? United have it in abundance.
As Wenger said: "At the moment we have to face sceptical attitudes from people." In comparison, in the wake of their comprehensive win over Schalke, United have been eulogised.
If March and April are the months where they advance, the time when they win the major matches, it has become the stage of the season where Arsenal habitually regress.
Permanently one year away from winning something, Wenger's focus is always deemed to be on the future. Ferguson captures trophies in the here and now while thinking long-term.
Javier Hernandez was a signing plucked straight out of Wenger's manual, an emerging player parachuted into the Premier League.
Yet it is quintessentially United that he does not decorate games as much as determine them. The Mexican is the grinning personification of the ruthlessness that the league leaders possess and that their distant challengers require.
It is why they are on course for the title. Hindsight should show this is Arsenal's great lost opportunity. For all the suggestions that they lack character, they are the team with the Premier League's best away record.
They have won at Aston Villa, Everton and Manchester City, whereas United limped away with a point on each occasion. But Arsenal cannot mimic United's relentless consistency at home, their near-immunity to shock results and their habit of taking something from games when they perform poorly.
The Londoners may deem this the season when Robin van Persie became a goal-a-game striker, when Samir Nasri's considerable talent came to fruition and when Jack Wilshere emerged at startling speed.
United, in contrast, can reflect on a campaign when Wayne Rooney was below his best for the majority of the time, when Dimitar Berbatov barely contributed away from home, when Park Ji-sung and Antonio Valencia were absent for three and six months, respectively, and when a 37-year-old, Giggs, increased in importance. And yet, in all probability, they still won the title.
Both clubs have been bywords for attacking football in their respective managers' long reigns, but the difference lies in defence; if Arsenal had the axis of Edwin van der Sar, Nemanja Vidic and Rio Ferdinand and United had employed four goalkeepers between the ages of 21 and 41, the likelihood is that the silverware would be bound for North London.
Instead, they were knocked out of contention for four trophies within two months. United are targeting the two most prestigious of all. Captaincy may be a burden for Fabregas, as Wenger argues, but it has raised Vidic's game to another level.
Rather than the on-field leaders, the variation in fortunes may be epitomised by the sights of the old foes on the touchline.
Wenger is a picture of frustrated helplessness, Ferguson is bullying his way to domination again. While the Frenchman names his strongest available side, the Scot can rotate his squad and retain his advantage. He has already confirmed that Giggs will be spared this game, with Anderson the likely replacement. Michael Carrick, a certainty to start, is relishing the crowded programme. "We thrive on it," he said. "We want to be playing in the big games and they're coming thick and fast now."
For Arsenal, matches assume an importance for other reasons. The title is gone, but Wenger said: "Even being in the top four is not done. We have to fight like mad."
True as that may be, the alternative approach would be to alleviate pressure by downplaying the danger Manchester City and Tottenham pose to his side.
A team who, in Wenger's words, are "wounded", are suffering from self-inflicted harm. Liberated by their release from a title race they found increasingly joyless, United's worry is they become more dangerous because of their failure.