Louis van Gaal's reign as Manchester United manager came under its most brutal, unforgiving scrutiny last night as his defence unravelled, his summer signings disappointed or departed, and his last hope of silverware this season disappeared. The inquest will be long and painful.
Only qualifying for the Champions League can rescue United's season.
Arsenal had their first win at Old Trafford since 2006, deservedly so, as they defended better than United, they showed more precision and intelligence in their attacking movements and took their goals well through Nacho Monreal and Danny Welbeck.
Wayne Rooney scored in between but it was Welbeck's strike, exploiting more hapless defending from Antonio Valencia, that settled it. United's hopes ebbed further when Angel Di Maria was sent off for diving and then reacting to Michael Oliver's decision by pulling the referee's shirt.
The eyes were always going to be on Welbeck. Many United supporters saluted the Arsenal goalscorer when he left the field in the second half, signalling respect and doubtless some frustration at the sale of a home-grown talent, especially as Radamel Falcao has struggled here.
Even before the goals went in, and before Di Maria was dismissed, bringing further noise from the terraces, Old Trafford had been rocking for fixture redolent of so many memorable FA Cup moments, including the epic finale of Alan Sunderland sliding in to score at Wembley in 1979, Nigel Winterburn taunting Brian McClair in the fifth round in 1988, Ryan Giggs's high-speed shimmy through Arsenal’s defence in the Villa Park semi-final of 1999 and Patrick Vieira's last-ever kick for Arsenal sealing the 2005 climax.
The usual niceties were ignored as usual. Marouane Fellaini went through midfield like a threshing machine, shredding Francis Coquelin as United started powerfully. Ashley Young began running at Hector Bellerin, troubling the young right-back, who was soon cautioned by Oliver. Coquelin began moving across to help Bellerin out. Bellerin's uncertainty was then exploited by Luke Shaw, gifting United the ball.
United were playing with a quick tempo, hounding Arsenal's back line, especially Per Mertesacker. Rooney, leading the line of United's 4-2-3-1 system, was quickly closing down Wojciech Szczesny.
Arsenal refused to be daunted. Their 9,000 fans gave them endless backing, giving Old Trafford such atmosphere. Arsene Wenger's men were defending deep, letting United build from the back, and happy to allow Chris Smalling to have possession. There were echoes of Arsenal's tactics across town at the Etihad on January 18, flooding midfield, Coquelin working overtime to protect the defence, and Santi Cazorla, scheming and tracking back on Daley Blind.
Cazorla was deeper here, alongside Coquelin, with Alexis Sanchez dribbling down the left, Mesut Ozil central and Alex Oxlade-Chamberlain speeding down the right. The movement of Arsenal’s small, creative types was too slick at times for Blind and Ander Herrera.
In omitting Olivier Giroud, Wenger had gambled on Welbeck being inspired on his return to the club he played for and still supports. Wenger hoped Welbeck's greater pace than Giroud would trouble Chris Smalling and Marcos Rojo, who covered well early on as Welbeck broke in. Welbeck was to punish United later on. Arsenal's nimble threat on the counter, and Valencia's erratic positioning, brought the visitors rich reward after 25 minutes.
Ozil picked out Oxlade-Chamberlain, who embarked on a magnificent slalom from right to left, beating Shaw, Smalling and Valencia. With United drawn across, space opened up on the left for Arsenal.
Oxlade-Chamberlain calmly slid the ball to the unmarked, untroubled Monreal, who took a touch and then curled the ball around the exposed David de Gea.
As Wenger did a little jig, as if negotiating a maypole, the 9,000 away fans chanted "one-nil to the Arsenal".
Up in the BBC studio, Roy Keane was delivering a withering verdict on the work of Blind and Valencia, barking that "the lack of character and commitment shown is just woeful. The lads have just downed tools there". He must have gone into meltdown at Valencia's later aberration. Valencia's travails provided another reason of why United are being so heavily linked with Southampton’s England right-back Nathaniel Clyne.
At least, Valencia's more assertive qualities were seen four minutes later, the Ecuadorean venturing down the right, pausing and letting Di Maria take over. The £59.9million Argentine's form has been so pored over. His body language since his house was burgled has been so scrutinised, but he was impressing here – until his response to Oliver's decision to book him in the second half for diving.
He had already been drilling in balls, looking for Fellaini. Here he targeted the run of Rooney, flighting the ball perfectly to clear Laurent Koscielny. Rooney finished with a thunderous header that gave Szczesny no chance. Rooney slid across the turf, celebrating his 12th United goal of the season, to add to his six for England.
Arsenal responded. They sensed the vulnerability in the hosts' defence.
Welbeck ran into the box, felt Rojo's left hand gently on his shoulder, and felt to earth, appealing loudly for a penalty as the Stretford End voiced its disapproval. Oliver waved play on. The England striker then rose above Smalling, heading goalwards but was denied by De Gea, who also clutched a Cazorla free-kick.
United hunted a second goal, a quick-fire move ending with Di Maria shooting straight at Szczesny, who then held a Fellaini drive. Van Gaal was clearly unhappy with the way his team were defending, and removed Shaw at the break, moving Rojo to left-back and inserting Phil Jones at centre-half.
Van Gaal also attempted to rectify United's lack of control in midfield by sending on Michael Carrick for Herrera, who had been booked and was failing to stifle Cazorla's darting runs from deep, as well as providing insufficient protection for United's defence.
Arsenal made a change when Oxlade-Chamberlain pulled a hamstring, bringing Aaron Ramsey on. The game swayed from end to end, Sanchez having a shot saved, Di Maria shooting wide and then United's defence dissolving again.
Again it was Valencia. He received a slightly awkward ball from the chest of Jones, but his reaction was madness, underhitting a back-pass.
Welbeck had anticipated well, seizing the ball, rounding De Gea and stroking the ball home. There was no mercy to his former club with his celebrations either.
De Gea kept the score down with an astonishing save from Cazorla, before United screamed that Bellerin should have been dismissed for another foul on Young. Di Maria did depart, punished for simulation and then given a second yellow card for grabbing Oliver's shirt.
Oliver then issued another caution for diving, waving the card at Adnan Januzaj, who went down far too easily when chased by Monreal.
Januzaj was incensed and risked following Di Maria down the tunnel.
It was all over bar the crowing, with Arsenal fans taunting their hosts, especially over Welbeck.
Arsenal are back at Wembley. United are back to square one. (© Daily Telegraph, London)