The early weeks of the season, when the 'Sarri-ball' philosophy that won him the adulation of peers such as Pep Guardiola brought optimism to Chelsea, seemed an age away as Manchester United breezed into the FA Cup quarter-final without needing to raise a gallop.

Sarri cut a solitary, dejected figure as his predictable moves and the inevitable results saw Stamford Bridge turn its fury on him with a force that begs the question not if his short time as Chelsea manager is drawing to a close, but when?

On this nightmarish evidence, and the fierce reaction of the supporters to their increasingly hapless and disconnected manager, the end may be very near.

Chelsea managers are wise not to book in for the long haul, even Premier League and Champions League winners, so it goes without saying that Sarri is a man clinging to his job in these reduced and struggling circumstances.

The rot was exposed by the 4-0 loss at Bournemouth and the 6-0 humiliation at Manchester City - their heaviest defeat for 28 years - but here they were embarrassed in their own home. And how the fans let Sarri know it as they loudly registered their disapproval.

As chairman Bruce Buck looked on with an expression set in concrete, it felt like the credits were already rolling for the manager who only arrived in July amid much fanfare.

Buck's bulletin to owner Roman Abramovich as he follows from afar will have been an ominous one. Sarri, barring an almost instant and unexpected upturn, now faces the fate that befell other high-profile managers - such as Luiz Felipe Scolari and Andre Villas-Boas - who were gone before their first season was out.

Chelsea's fans have already decided. Sarri has lost them.

And once a fanbase is lost, it usually takes something out of the ordinary to win them back, something Sarri shows no signs of delivering.

As Chelsea failed to respond to first-half goals from Ander Herrera and Paul Pogba that put United in complete command, the atmosphere turned toxic.

Sarri has not, and does not, help himself with a stubborn refusal to change tack to such an extent his substitutions are now so predictable they are greeted with sarcastic applause and the sound of laughter.

When, with grim inevitability, Mateo Kovacic was yet again replaced by Ross Barkley, there was a burst of ironic cheering followed by the chant of "you don't know what you're doing".

Yet this is what Sarri does, repeatedly, with diminishing returns. Inflexible, stubborn and - to the fans at least - infuriating.

He is the master of the like-for-like substitution - all very well if the results are good, but managerial madness when Chelsea end up well beaten again.

'Sarri-ball' - which is meant to be high-paced attacking football combining pressing and short, quick, passing exchanges - has turned out to be tedious, predictable and easily rumbled by teams of higher quality.

Jorginho, very much Sarri's man as he arrived from Napoli on the same day as his appointment, is the symbol of its shortcomings. Once teams worked out he could be negated by being crowded out and blocked off, much of the game was up.

Same Sarri story. Same Sarri result.

Jorginho's struggles are only emphasised further by the fact the uncomplaining N'Golo Kante, widely regarded as the world's best midfield anchor, has been shunted to the margins to accommodate an inferior player who wins Sarri's favour. It is a point of serious contention, and a valid one. It makes no sense and yet Sarri persists.

Chelsea's fans even called for the return of Frank Lampard, who is in his managerial infancy at Derby County, before Sarri was treated with arguably the most dangerous reaction of all - mockery.

As United's supporters chanted "you're getting sacked in the morning", it was taken up by Chelsea's fans and swept loudly around Stamford Bridge.

It was a significant moment and a measure of just how badly Sarri is currently regarded by his own supporters. He has not received this sort of hostile treatment before but this was the dam of frustration bursting and he was the catalyst.

Sarri's body language was hardly encouraging, agitated from the first whistle, increasingly frustrated as Chelsea slid further into the abyss and in the end gesturing angrily at his own players as they were reduced to a succession of aimless long balls.

It was a night that had all the feeling of the end of an era, albeit a short one, although it would be incredibly bold, even for Chelsea, to sack a manager as they prepare to meet Malmo in the Europa League on Thursday followed by the Carabao Cup final against Manchester City at Wembley on Sunday.

Sarri is trying the patience of an owner not known for that particular commodity and, presuming he survives, the acid test will surely come against City, with a Premier League game against Tottenham at Stamford Bridge three days later.

The Italian recently questioned his own ability to motivate Chelsea's players after the lacklustre defeat at Arsenal and questions about his methods and whether they were cutting through became more pointed after the humiliation by Manchester City.

This loss, a stroll for United once they went ahead, only drove deeper into the concerns about Sarri's approach.

Chelsea's players look lost, short of inspiration and devoid of ideas. The big worry for Sarri and Chelsea's hierarchy is he looks exactly the same.

Sarri said Chelsea played "confused" football in the second half. There is a reason for this - the gameplan looks confused and the manager looks confused, so it is hardly a huge surprise the outcome is confused.

Every detail of this night will be relayed back to Abramovich who, despite the distance, will be well aware of just how restless the natives are.

It is hard to see Sarri coming out of this crisis unscathed.