A fortnight ago, Gunners fans were talking optimistically of a Quadruple.
Now they have only the Barclays Premier League to play for after goals from Fabio and Wayne Rooney either side of the interval sent United into the semi-finals.
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.
The backdrop to this fixture could hardly have been more gloomy.
Between them over the past couple of weeks, two of England's biggest clubs have lost two Premier League games, a cup final and a Champions League tie.
They have had a player sent off who shouldn't have been, not seen an opponent reduced to 10 men when they should have and imposed a blanket media ban.
In addition, Ferguson and Wenger are both facing the wrath of a governing body for comments about match officials that went beyond acceptable levels.
On that basis, neither Manchester United nor Arsenal would really have wanted to play the other knowing defeat would pitch a gloomy period into a crisis.
Not that you could tell from Ferguson's team selection.
Seven defenders for a game at Old Trafford is hardly the norm for a man who has never been scared of attacking the best opposition.
However, as so often in the past, Ferguson clearly knows far more about his players than anyone else.
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 when he met Fabio's cross with a header he was unable to keep down.
There was no such escape for Arsenal when Fabio surged forward later on, 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. 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 very 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.
The onslaught did not last. Or at least it 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 making his re-appearance after 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 outstanding 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.