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 Solskjaer, came through. United would rally 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 Solskjaer 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 who had presided over a historic humiliation. Never in the Premier League era had the club been four goals down by the break at home.

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 – his third of another hay-making day – and the only relief for Solskjaer was that the jeers at full time were not as vitriolic. Most of his detractors had long since left the ground. United had played out the final 30 minutes with 10 men after Paul Pogba, on as a half-time substitute, was sent off for jumping into a nasty tackle on Naby Keita, which saw the Liverpool player taken off on a stretcher.

Liverpool were excellent. Again. They remain unbeaten this season and the statistics show that they have scored a minimum of three goals in every away match, 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.

But, as strange as it sounds, Liverpool did not have to locate their highest gears. They almost strolled to victory, Solskjaer’s game plan was shredded early on and some United fans were heading for the exit in the 38th minute when Salah added to goals from Keita and Diogo Jota to make it 3-0.

Sir Alex Ferguson could be seen in the stands, shaking his head slowly, and the only question on everybody lips was whether Solskjaer 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. The faith of the club’s board in Solskjaer will be tested as never before.

He had gone for the double pivot of Fred and Scott McTominay, mindful that he did not want to be too open in the middle of the pitch, but United were exactly that. As against Atalanta in the first half, the front four – including Bruno Fernandes – seemed a long way away and Liverpool were able to make their extra man in midfield count.

They were able to do pretty much whatever they wanted during a first half that made the United fanbase squirm with embarrassment. Time and again, Liverpool were allowed to play up through the United lines – and it was an invitation that they accepted with glee. Where were the challenges, the pressure on the player in possession?

There are many reasons why United have kept only one clean sheet in 21 matches and their poor organisation and poorer decision-making were stamped all over the first-half concessions,. It began with the first when Luke Shaw was too deep, making United’s last line wonky and playing Salah onside when Roberto Firmino looked forward for him. Salah did not need to produce any magic, just a simple pass around Shaw for the overlapping Keita, who side-footed past David de Gea.

The game might have been shaped differently had Fernandes not fluffed a glorious chance on four minutes, snatching at his shot and sending it well wide after United had worked the ball nicely from left to right, with Mason Greenwood playing the final pass.

It was downhill for them, at pace, thereafter, Liverpool scenting blood and gorging on it. The second was the worst of the first-half bunch from a United point of view, Harry Maguire getting himself into a dreadful tangle with Shaw on the edge of the United area and allowing Keita to collect and go wide for Trent Alexander-Arnold. He crossed low and hard; Jota, who had been preferred to Sadio Mane, slid in at the far post to score.

United had a few moments in the final third before the interval. It was just that when they had to defend their supporters were left to watch through their fingers. Liverpool were quicker and smarter. They seemed to want it more. United were increasingly gripped by fear, the resistance minimal.

It almost went under the radar in the first half that Firmino missed two great chances and De Gea made a save to deny Salah. When Salah flicked in the third, following a Keita 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 Greenwood pass and he kicked out at Curtis Jones, who had come on for the injured James Milner. Ronaldo was booked. Liverpool responded by working yet another opening – almost casually – which Salah finished off from Jota’s pass.

Solskjaer 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 on to Jordan Henderson’s magnificent through-ball to score the fifth and it was all too 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.