By the time this game had finished Ederson, Manchester City’s goalkeeper, had just reappeared from the x-ray suite at the clinic where he had been taken to check his cheekbone and jaw were still in working order. Ederson had been caught flush in the face, head-high, by Sadio Mané’s flying studs and the red card that was shown to the Liverpool player does at least offer his team some kind of excuse for this score.

Not entirely, though. There is a way of playing with 10 men and this was absolutely not it. Liverpool’s response to Mané’s dismissal did not offer great encouragement about their competitive courage and, not for the first time, it was difficult to imagine how Jürgen Klopp can achieve his ambitions if his side are going to defend this generously. Liverpool crumbled far too easily and once City sense weakness they can be merciless opponents. It was the first time City have scored four, or more, past the team from Anfield since 1937 – and Liverpool’s heaviest defeat in Manchester for 70 years.

City were already winning, courtesy of Sergio Agüero, by the time the referee, Jon Moss, decided that Mané’s collision with Ederson 25 yards from goal constituted an automatic sending-off eight minutes before half-time. Gabriel Jesus headed in a second in the time added on at the end of the first half and when the same player made it 3-0 early in the second half it was almost a surprise City restricted themselves to two more. Leroy Sané, one of their second-half substitutes, scored them both and Ederson had just arrived back in the stadium to see the final one – the game’s outstanding moment – fly into the top corner.

This was certainly not the way Alex Oxlade-Chamberlain, a substitute, must have imagined his Liverpool debut going and the punishment for Mané will be considerable, with a three-match ban to follow. Will Liverpool appeal? Possibly, but it is unlikely that the decision will be overturned. These matters are not decided on whether or not it was intentional. Klopp’s reaction was that it would be “another waste of time”.

As red cards go this one will polarise opinion. Mané’s sympathisers will argue that he was entitled to go for the ball and that, as Guardiola conceded, it was an accidental collision. Equally, the attacker’s studs were way off the ground if the argument is whether it constituted dangerous play. All that can really be said for certain is that it poleaxed Ederson and left a clutch of City players frantically waving for medical assistance. The goalkeeper had taken an almighty whack, requiring eight minutes of intensive treatment, and it was lamentable that some Liverpool fans seemed to hold him responsible, booing as the stretcher bearers prepared to take him off.

Sadio Mané catches Ederson in the face, resulting in a red card for the Liverpool player. Photograph: Lee Smith/Action Images via Reuters

A goal behind and a man down, it was always going to be a challenging and potentially treacherous afternoon for Liverpool from that point. Even so, it was startling to see the way they capitulated. Klopp’s team seemed unsettled, to say the least, as soon as play restarted. They had little or no idea about what was required for a side in that position and, unfortunately for them, they were not playing a team that was going to exhibit any form of pity.

A linesman’s flag saved the visitors, two minutes into first-half stoppage time, when Kevin De Bruyne crossed from the right and Jesus sent a header past Simon Mignolet. The next time De Bruyne picked out the Brazilian, this time from the left, he was onside and unmarked again. Mignolet was hopelessly exposed and, regardless of what had happened with Mané, the problem for Liverpool here was the absence of defending and leadership.

Early on, the same could be said of their opponents bearing in mind there were long parts of the first half, at 11 v 11, when the home team had seemed intent on providing hard evidence that Pep Guardiola had been neglecting defensive work. Not for the first time, Nicolás Otamendi in particular looked susceptible. Mohamed Salah gave him a torrid time in the opening exchanges but the Egyptian was withdrawn after Mané’s red card and the second half was a fun, stress-free 45 minutes for the home side.

Liverpool’s carelessness could be summed up by the stray pass from Jordan Henderson that led to City’s third goal, with Fernandinho’s through ball leaving Agüero running free. The Argentinian had been in this position for the opening goal, picked out by De Bruyne’s brilliantly measured pass, and chose on that occasion to take the ball around Mignolet. Now, however, he had Jesus in support and rolled the ball sideways to leave his team-mate with a straightforward finish.

The last half-hour was an exercise in damage limitation for Liverpool but there was still a distinct lack of evidence that they knew how to keep it tight and make sure the afternoon did not become even more of an ordeal. Sané’s first goal followed a slick exchange of passes with Benjamin Mendy on the left and the best was saved for last as the substitute let fly from 25 yards to send a curling shot into the top right corner of Mignolet’s net.