Chelsea will not stand for much more of this. Maurizio Sarri, isolated and alone, shuffled around his technical area as his team were outmuscled, outmanoeuvred and outclassed by Manchester United to depart the FA Cup with the kind of whimper that has become far too commonplace over recent weeks. The Italian’s eyes never left the pitch but he could not have escaped the full repertoire of chants, whipped up by the fans in the Matthew Harding stand – where faith in the current regime appears wrecked beyond repair – and which damned their own head coach. The humiliation was brutal.

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The chorus veered from “You don’t know what you’re doing” at the predictability of the substitutions, via an industrial and scathing assessment of Sarri-ball, a style this team have hardly ever mustered and clearly never mastered, to bellowed praise of Frank Lampard. Chelsea’s record goalscorer has other priorities at present, his focus on securing Derby’s elevation from the Championship, but his candidacy has arguably been enhanced by the startling success being enjoyed by Ole Gunnar Solksjær since he returned to Old Trafford in December. United stride on into a quarter-final at Wolverhampton. Sarri’s own future appears distinctly less certain.

In the west stand Roman Abramovich’s box remained dark and empty, with the owner presumably oblivious to the cries of “You’re getting sacked in the morning” that were initiated in the away end but swiftly echoed around the arena’s four sides.

The oligarch’s absence is adding to the sense this is a club that is rudderless at present. In the recent ruthless past he might already have acted. Perhaps after the embarrassing defeat at Bournemouth at the end of January or, more probably, the abject capitulation to Manchester City at the Etihad stadium.

Victories over Huddersfield and Malmö secured only temporary respite. There is too much wrong with this setup, from the stubbornness of a coach unwilling to bend his philosophy, once considered a strength, to suit personnel to players whose confidence is shattered or whose commitment is questionable.

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John Terry was here watching and he must have winced at the sight of Chelsea wilting in the face of United’s pace, power and sheer aggression. David Luiz and Marcos Alonso squabbled through the aftermath of the first goal, while too many home players were left waving their arms in baffled submission as they digested the loss of the second.

Admirable as Solskjær’s revived team were, that traumatic evening against Paris Saint-Germain apparently forgotten, the holders had made it easy for them, just as they have for far too many opponents of late.

Sarri has the return leg with Malmö to come on Thursday, then the Carabao Cup final against City and a visit from Tottenham, all within the next eight days. The thought of that occasion at Wembley must fill him with dread, given how easily his players are pulled out of shape at present, although in view of the current mutinous mood he could yet be spared involvement.

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His assessment that Chelsea had been unlucky in the first half – “when we played better than the opponent” – was questionable. The spikiness had always been United’s, whether they were snapping into tackles – some illegal – to knock their hosts off their rhythm or niggling at Chelsea to leave them flustered.

Even Chris Smalling summoned a shove, unnoticed by the officials, which sent Alonso careering into the advertising hoardings midway through the opening period, a sly nudge that rather summed up their resolve.

United were never likely to wilt but they knew Chelsea could be knocked off their stride. The victory was established by the interval. Theirs had been the aggression, the energy and the quality to bully fragile players into submission.

Perhaps the hosts may have settled had Gonzalo Higuaín guided a free header on target or Sergio Romero not thwarted David Luiz’s free-kick and Pedro’s follow-up. Instead the home side were breached and all the insecurities exposed at the Etihad and Vitality stadiums reared again.

Juan Mata’s clever shuffling of possession through midfield had helped conjure their opener, freeing the outstanding Paul Pogba from the mess of bodies down the flank. He teased space from Antonio Rüdiger before sending in a glorious cross to the far post where Ander Herrera was free to nod back across Kepa Arrizabalaga and into the far corner.

Facebook Twitter Pinterest Ander Herrera heads in a pinpoint cross from Paul Poba to put Manchester United ahead. Photograph: John Sibley/Action Images via Reuters

At first glance it seemed inconceivable that the diminutive Spaniard had been permitted to leap and score by Alonso, the fall-guy of the moment. Yet Chelsea’s entire midfield had been attracted to Mata’s promptings with Mateo Kovacic, who might otherwise have tracked the Spaniard’s run in the buildup, leaving the whole formation lopsided.

César Azpilicueta, too, had been caught upfield and was still ambling back as the net bulged, with the on-field captain looking increasingly exhausted by his regular toils.

As half-time approached, Chelsea surrendered the ball yet again just inside their own territory for Pogba to slide Marcus Rashford into space down the right. A panicked David Luiz dangled a leg at the England forward, whose cross was duly met emphatically by Pogba, diving between Rüdiger and Azpilicueta in the centre, to double the lead via the goalkeeper’s right glove.

Facebook Twitter Pinterest Gonzalo Higuaín, Jorginho and Mateo Kovacic sum up the mood of the Chelsea fans after Manchester United’s opening goal. Photograph: David Klein/Reuters

Everything his team managed thereafter was confused, according to Sarri, in forlorn pursuit of parity as the mood became increasingly poisonous and his decision to throw on Davide Zappacosta rather than Callum Hudson-Odoi, for whom the clamour had been deafening, felt like two fingers being stuck up at the crowd. With that relationship fractured, time is running out for the Italian.