When the half-time whistle sounded, with Liverpool 4-0 up and everybody inside Old Trafford well aware that it could have been seven, the boos from the Manchester United fans rang out.

At half-time against Atalanta in the Champions League last Wednesday night, with United 2-0 down, there had been a volley of jeers before the support for the team and the manager, Ole Gunnar Solskjær, came through. United rallied to win 3-2.

Not here. The boos were sustained in their intensity, the anger bubbling, and it was certainly a long and lonely walk for Solskjær back along the touchline and into the tunnel. At that moment, it felt as though we were witnessing the end of a manager who is well liked by the United diehards but has struggled to keep his head above increasingly choppy waters this season. Never in the Premier League era had the club been four goals down at home by the break.

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The second half was played out to a soundtrack of goading from the travelling Liverpool fans. They chortled about Ole being at the wheel and chanted that he must stay. Their requests for a wave from him went unheeded.

Mohamed Salah scored his team’s fifth on 50 minutes – his third of another hay-making day – and at that point it seemed as if the final scoreline could have been anything. The feeling gathered pace when Paul Pogba, on as a half-time substitute for United, was sent off just after the hour for jumping into a nasty tackle on Naby Keïta. The Liverpool midfielder was carried off on a stretcher.

Liverpool, though, left it at that, which was a relief for Solskjær, as was the fact that the jeers from the crowd at full-time were not as vitriolic. Most of the Norwegian’s detractors had long since left the ground.

Liverpool were excellent. Again. They remain unbeaten this season and the statistics show they have scored a minimum of three goals in every away game, with Salah’s hat‑trick continuing his club‑record scoring run. It is now an astonishing 10 matches on the spin in which he has found the net.

For Solskjær and United, the humiliation was total. Liverpool pressed brilliantly and they were ruthless in the final third, shredding Solskjær’s gameplan early on, and some United fans were heading for the exits in the 38th minute when Salah added to goals from Keïta and Diogo Jota to make it 3-0.

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Sir Alex Ferguson was pictured in the stands, shaking his head slowly, and the only question on everybody lips was whether Solskjær could survive from here. United find themselves eight points behind the league leaders, Chelsea; the hoped-for title challenge in tatters before the clocks go back.

It was not as if this result had come out of nowhere, with United having got away with it on several occasions this season. But for David De Gea saves at Southampton, Wolves and West Ham, the team would already have been down a fistful of points. This was the day when everything fell apart and, after what was a heaviest home defeat against Liverpool, the board’s faith in Solskjær will be tested as never before.

Solskjær’s double pivot of Fred and Scott McTominay was overwhelmed, with Liverpool able to do pretty much whatever they wanted during a first-half that made the United fanbase squirm with embarrassment. Time and again, the visitors played up through the United lines. Where were the challenges, the pressure on the player in possession?

The game might have been shaped differently had Bruno Fernandes not fluffed a glorious chance on four minutes, snatching at his shot when well placed, and Liverpool piled on the agony thereafter.

Keïta’s opener came after Luke Shaw had played Salah onside – the first sign that United’s last line was all over the place. Salah’s assist was simple and so was the finish and the second was probably the worst of the lot from a United point of view.

Harry Maguire got himself into a dreadful tangle with Shaw on the edge of the United area and it allowed Keïta to go wide for Trent Alexander-Arnold. He crossed low and hard; Jota, preferred to Sadio Mané, slid in unmarked at the far post.

When United had to defend, their supporters were left to watch through their fingers. Liverpool were quicker and smarter, and it almost went under the radar that Roberto Firmino missed two great chances in the first half and De Gea made a save to deny Salah.

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When Salah flicked in the third, following a Keïta cross and more statuesque defending, Sir Kenny Dalglish and Ian Rush could be seen almost doubled over with laughter in the posh seats.

Cristiano Ronaldo lost his head after he failed to beat Alisson from a Mason Greenwood pass and he kicked out at Curtis Jones, who had come on for the injured James Milner. Ronaldo was shown a yellow card. Liverpool responded by working yet another opening, which Salah finished off from Jota’s pass.

Solskjær just wanted it to end – a darkly comic detail coming on 53 minutes when VAR ruled out a Ronaldo finish for a hairline offside. By then, Salah had run onto Jordan Henderson’s magnificent throughball to score again and it was all much for Pogba, who was dismissed after a VAR review.

De Gea would make a fine save to deny Alexander-Arnold. United had suffered enough.