In the end humiliation by their fiercest rivals was the final straw.
Because Manchester United weren’t just beaten by Liverpool at Anfield on Sunday, they suffered unedifying, excoriating humiliation.
And not for the first time, Anfield became a graveyard for Jose Mourinho’s dreams and ambitions.
In 2005, following Luis Garcia’s ‘ghost-goal’, the then Chelsea manager told how he: “felt the power of Anfield, it was magnificent.”
That Champions League semi-final hurt, but that spring Mourinho had the significant consolation of having celebrated a first Chelsea league title for half-a-century.
Fast forward 13 years and this faded shell of a once peerless continental coach felt the power of a revitalised Liverpool team and fan-base in irresistible harness.
It is not a coincidence that defeats by Jurgen Klopp teams have now seen Mourinho sacked on three separate occasions.
Because while the Portuguese preaches agitation and antagonism, a creative tension which can produce short-term benefits, Liverpool’s German coach is an advocate of togetherness and harmony – a bond he hopes will bring long-term success.
And that unity at Anfield on Sunday was simply too much for Mourinho to handle.
Not since Manchester United were relegated to the Second Division in 1974 has a Liverpool team proved so superior to their fiercest rivals.
And even Mourinho accepted that fact.
“You can compare my Porto team with Liverpool,” he said.
“They are fast, they are intense, they are aggressive, they are physical, they have an objective.
“They play 200 miles per hour with and without the ball. I am still tired just looking at Robertson. He makes 100 metre sprints per minute, absolutely incredible.”
United, on the other hand, were stodgy and sluggish.
Liverpool played at a different pace, a different intensity, a different level to United.
They recorded 36 shots – the most, according to Opta, that United have ever faced in a Premier League game.
And it was too much for Mourinho to bear.
Schadenfreude is that peculiar German phrase which explains the state of deriving pleasure from another person’s misfortune.
The schadenfreude Reds fans enjoyed on Sunday night will soon dissipate as United start the search for a modern manager who may guide United back to the upper echelons of the Premier League again.
But they won’t do it any time soon. United have a long way to go to catch Liverpool.
Jose Mourinho has felt the power of Anfield once again.