In Arsène We Trust. Not here. Not after the 73rd minute, when Arsène Wenger made a substitution that had even his captain Robin van Persie mouthing his disbelief. Alex Oxlade-Chamberlain had just set up Van Persie's equaliser and a stirring second-half jump back from the brink by Arsenal looked set to yield something tangible.

Yet Wenger's decision to replace Oxlade‑Chamerlain's youthful cut and thrust with Andrey Arshavin, who is the player that Arsenal fans love to hate, brought jeers and derision in virtually all corners. And to give the game's principal sub-plot its savage pay-off, Arshavin just had to showcase some of his weak defensive tracking on Manchester United's winning goal, swept home by Danny Welbeck.

Wenger hurled his water bottle down in disgust but behind him in the stands, there were vociferous shows of dissent. Some Arsenal supporters stood to applaud him sarcastically and, later on, there were cries for him to "spend some fucking money", which has become the default setting at times of soul-searching.

At full-time, as the Sky TV cameramen hurried their lenses towards the Frenchman, there was the sight of Sir Alex Ferguson sending him down the tunnel with a sympathetic pat on the back. The Emirates does not normally teem with such open hostility towards Wenger, the manager whose work for the club is so highly regarded, but this felt like a dreadfully low ebb.

The story for Arsenal might have centred upon Van Persie's 23rd goal from 28 appearances this season and the tribute he unveiled on his vest to his grandfather, who was celebrating his 91st birthday in the crowd.

It could have taken in the flashes of promise that Oxlade-Chamberlain showed or, above all, the extraordinary comeback that those in red produced after a first‑half display marked by nervousness and basic errors.

Instead, after Arsenal's third Premier League defeat in succession, it became all about Wenger. On the eve of the game, he had joked that he did not "stay sane" in the face of the pressures that he faced. Plenty of Arsenal fans left the stadium wondering whether he had cracked up.

He had not. He had just made a mistake, despite his determination not to admit it. And the wider question has to be asked concerning Arsenal's fragility. The home team were firmly in the ascendancy and the withdrawal of a tiring teenager on his full Premier League debut ought not to have checked their momentum.

Nor should the introduction of the Russia captain, however disinterested Arshavin frequently looks and however much scorn he invites. Up in the Sky studio, Gary Neville seized the opportunity to plough into the back of him, as he used to do to Jose Reyes.

The episode felt symptomatic of Arsenal's knife-edge existence and even these 90 minutes contained wild mood swings. Booed off at half-time, the jeers returned with interest upon the final whistle but, for the majority of the second half, the home crowd could be delighted at how their team had played.

Everybody inside the ground, chief among them Wenger, suspected that it would not be Arsenal's day when Van Persie belted a golden chance wide in the 51st minute, after Chris Smalling had slipped. Yet Arsenal carried the fight and they created three decent openings before Van Persie atoned with his threaded shot, which proved marginally stronger than Anders Lindegaard's fingertips.

United looked more assured, more balanced and, as even Wenger admitted, "a bit more mature in every position than we are." A glance at the substitutes' benches told you all you needed to know about the strength in depth at both clubs at present.

While United boasted big reputations, Wenger might have counted three players that he wanted to sell; another that he wished he had not signed in the summer; two unproven teenagers and Yossi Benayoun. Wenger pleaded for understanding over his 11 absentees.

Arsenal's first-half travails were epitomised by Johan Djourou, one of the central defenders who has been asked in recent weeks to stick his finger in the defensive dam. The stand-in right-back has found it overwhelming. Sent off at Fulham in the club's first fixture of 2012, he was dominated by Nani and fortunate that the winger's end product was disappointing. Eventually, United reaped reward from their inroads up the left when Ryan Giggs crossed for Antonio Valencia, who exposed Thomas Vermaelen, another centre-half filling in at full-back. The home crowd cheered the announcement that Djourou would not reappear for the second-half.

Wenger was defiant afterwards, and protective of his players. No individual would be singled out for blame. Not even Arshavin. Things would improve, he insisted, when his injury-list eased. Spending in January was still not the answer.

Yet this was a game that Wenger admitted his team "could not afford to lose" in the fight to finish fourth. United continued to pile on the misery. After the 8-2 at Old Trafford in August, this was a 10th victory for them over Arsenal in 13 meetings.

If Wenger thought his season had hit rock bottom in the traumatic early weeks, he had better think again.