A fortnight ago, Gunners fans were talking optimistically of a Quadruple.
Now they have only the Premier League to play for.
Arsene Wenger at least has a week to lift his troops before they are in action again but the mental scars of Arsenal's catastrophic collapse may not heal easily as they seek to overhaul a United side who ended their own trauma in perfect style ahead of their Champions League meeting with Marseille.
Sir Alex Ferguson sprung a surprise by naming seven defenders in his starting line-up - Fabio and Rafael started on either wing, while John O'Shea played in midfield with Darron Gibson - yet the performance was arguably one of United's best of recent times.
In truth, Brazilian twins Fabio and Rafael have often caught the eye offensively, even though they have been occupying full-back berths.
Given the opportunity to express their ability from more advanced positions, knowing Rooney had dropped back to fill a midfield hole, they duly did just that.
Rafael could easily have put United ahead early on when he met Fabio's cross with a header he was unable to keep down.
There was no such escape for Arsenal in the 28th minute, though, when Fabio surged forward, setting in motion a move that ended with a smart Rooney cross for Javier Hernandez.
The Mexican's header was aimed for the far corner and Manuel Almunia could do no better than palm it away, straight to Fabio, who slid in and found the roof of the Arsenal net.
On the touchline, Wenger must have had an awful sinking feeling, knowing another trophy was being snatched away from him after Birmingham and Barcelona had snuffed out their hopes on two other fronts.
Skippering a strong line-up, Robin van Persie saw a curling shot turned round the post by fellow Dutchman Edwin van der Sar, the striker then heading the corner narrowly wide.
Just before the interval, Samir Nasri fired a precise shot through Wes Brown's legs, forcing Van der Sar into another excellent save.
Generally though, it had been a listless display from the Gunners, leaving Wenger with a job on his hands to revive spirits.
Initially, it appeared the Frenchman's words had produced a positive effect.
Van der Sar was twice called upon to keep Arsenal out, first when Wes Brown turned the ball goalwards as he tried to intercept a Laurent Koscielny cut-back, the second when the Frenchman curled a shot to the far corner with the rebound.
But the onslaught was immediately halted by a second United goal.
In sticking out a leg to stop Rafael's cross, Johan Djourou could only loop the ball high into the air.
Rooney escaped Sagna's attentions and although his header lacked power, the direction was perfect, far enough away from Almunia but not too far to prevent him finding the corner from an acute angle.
Antonio Valencia had played a part in the move after replacing Fabio at half-time, his first appearance since sustaining a serious ankle injury in September.
The Ecuador star diligently patrolled his wing as Arsenal threw caution to the wind.
But with Van der Sar producing four good saves to deny the visitors, one to repel substitute Marouane Chamakh, who had been presented with a free header by Sagna, there was no way back for the shellshocked Gunners.
In fact, their evening got even worse as they ended the game with 10 men after Djourou had been stretchered off following a nasty collision with Sagna.