As embarrassing and as painful as it must have been, it was not the defeat at a ground where David Moyes established his Premier League coaching credentials that killed his hopes of being allowed a second season at Old Trafford.
For United's key decision makers, Moyes has been a dead man talking for at least a month.
Nor was the avalanche of unacceptable results - the snowballing list of unwanted Premier League-era lows - the critical factor in electing to dismiss a manager who turns 51 later this week. What likely undid Moyes was the damage his man management was inflicting upon United's bottom line.
Time and time again this season Moyes has sought to lay the blame for the club's unprecedented under-performance – from champions by 11 points to eliminated from all domestic contention by the end of January – on the squad he inherited from Sir Alex Ferguson. And as the executive vice-chairman promoted alongside him last summer encouraged Moyes' efforts in overhauling United's scouting department while telling public and press that the replacement Scot's job was safe, he grew confident of an immense summer investment in new players.
Discussion of a 100 million pound-plus spend on transfer fees alone became as common as a Moyes press conference emphasizing the dire need to restructure. His recruitment plan involved the extraordinary measure for an elite European club of signing as many as six top-class players in the peak period of their careers, namely over the age of 25. And all this on top of the near 65 million pounds in transfer fees already thrown at Marouane Fellaini and Juan Mata, plus a club-record contract controversially awarded to Wayne Rooney.
In eight years of ownership by the Glazer family, Ferguson's net transfer spend averaged out at under 20 million pounds per campaign. The strategy Moyes proposed to turn his own failing results around was that he be entrusted with more than five seasons worth of such expenditure in one single summer splurge.
If such logic was never likely to appeal to owners whose interest in football has always been about profit rather than pleasure, Moyes' ability to destroy value in the players he'd been hired to work with counted heaviest against him.
Amongst Moyes' first requests as United's manager was that Nani be retained on a contract that made him one of the club's very best paid professionals. Less than a month into their first Premier League campaign together, Moyes had fallen out with the Portugal international and ultimately asked that he be sold. In total, the Scot has granted Nani just six league starts.
Ashley Young has 12, Shinji Kagawa 11, Javier Hernandez five. All are in what should be the most productive periods of their careers. All were regarded as prime assets under Ferguson. All have diminished - perhaps plummeted - in value as Moyes has continuously kept them sidelined throughout the season.
In his handling of Rooney, Moyes undermined the team in a different manner. Pushed towards the periphery by Ferguson over his attitude to training, lifestyle and the club that had made him one of England's wealthiest sportsmen, Rooney wanted out as United's new manager was arriving.
Moyes correctly calculated that the striker could be retained by making him feel special again. No one had been more important than Robin van Persie to Ferguson's final league title, yet it was Rooney who was to be the central focus of United's new and decidedly unimproved attack; his treatment around the training ground giving evidence to the Englishman's restored status.
As it became clear that an individual who'd fallen afoul of Ferguson and twice backed United into a corner over new contracts was to be rewarded with the most lucrative deal the club had ever paid, discontent with Moyes grew. The word amongst senior players was that their new boss simply did not understand the essence of United as a football club.
In his promotion of Rooney, his team selections, his overly cautious tactics, his man management and his words, they saw a man contradicting the principles Ferguson had worked so diligently to instil within them. Gradually, the unease of senior players – Ryan Giggs notable amongst them – turned into a consensus that Moyes was the wrong man for the job. Ultimately, representations were made to the board that a change of manager was essential.
United's hierarchy was already thinking the same way. By March, the sensible precaution of assessing candidates to replace Moyes had begun. By the end of that month, the word within the club was that Moyes was "dead" and that the six-year contract with which the Glazers had sought to bolster the first managerial appointment since they purchased the club would have its unpublicized break clause activated by the summer.