Right flank folds
There was no doubt which side City had identified as United's most vulnerable. They targeted the visitors' right flank virtually from the off, with Diogo Dalot picking up a yellow card after 95 seconds for chopping down Jack Grealish.
Things never got any better. With Dalot now knowing that one more mistake would lead to a red card, City could just keep on attacking that side with impunity, and United's right-back was certainly not helped by Antony being unable to get into the game even in a defensive capacity, and only getting on the ball in a forward position when City were already 4-0 up.
Grealish enjoyed most success in that area of the pitch but Bernardo Silva also moved into pockets of space and set up the opening goal for Phil Foden. By the time Foden scored his second goal, also from that area, there was a huge amount of space for City to attack.
Ultimately, the numbers told their own story: City launched 42.6 per cent of their attacks down that side of the pitch.
No Casemiro and conceding midfield
Casemiro was United's big summer signing - a five-time Champions League winner, and three-time La Liga champion with Real Madrid, with just the kind of big game experience that United needed in a game that always threatened to be a huge test of their recent upturn in form.
And yet where was Casemiro when the game kicked off? Sitting on the bench alongside Cristiano Ronaldo. It was a curious decision, not least because City were missing their usual midfield metronome, Rodri. And while the Brazilian was sidelined, Kevin De Bruyne was allowed to run the game in midfield.
Ten Hag waited until almost the hour mark before bringing on the 30-year-old but, by that time, City had taken advantage of the mismatch. Scott McTominay, selected to start ahead of Casemiro, had a poor game and it all meant United were unable to compete with City’s midfielders, especially with Joao Cancelo drifting into the middle from his starting position of left-back to overload the central area.
Gary Neville accused United's players of "bottling" the match and surely the sight that will infuriate Ten Hag more than anything was his players ambling as City passed the ball around them with such intent. Even in their own territory, City players got on the ball and looked purposeful.
By contrast, United were jogging, hoping that City might lose possession. United were playing against a team that did not allow them to lay a punch, so it was different to Brentford. But this was back to those dark days when heads went down and Ten Hag had them in at Carrington the following day to make up the difference in kilometres from their opponents.
At least Marcus Rashford and Jadon Sancho sprinted down the tunnel at half-time, showing they were willing to show some pride in the second half - the rest trudged off and looked beaten.
When Raphael Varane went down injured in the first half, at 2-0, he looked ready to carry on after treatment. City scored from the corner when he was off the field of play - Christian Eriksen and McTominay ended up marking Erling Haaland, with predictable results - and then he was subbed soon after that, sitting on the bench rather than heading down the tunnel for treatment.
Another symptom of the lack of intensity was United's carelessness on the ball - a point exemplified by Bruno Fernandes, who continually misplaced passes.
Where were the changes?
It was clear that United were being overwhelmed by City within minutes, even before the floodgates opened, and Ten Hag’s game management has to be called into question.
At 2-0 there was time to make changes, even if it meant hauling off a player barely half-an-hour into the game. Casemiro sat on the bench for too long but Ten Hag could also have changed his system as the right flank was so badly exposed.
It was not until almost the hour mark that he made changes to his forward line, after Luke Shaw was introduced at the interval. United’s late goals diluted some of their embarrassment but they only came after Pep Guardiola had given a breather to the players who were running the game for him.
Lack of a striker
Finding a suitable No9 over the summer was one of the priorities for Ten Hag but United could not land a striker, which has left them with a problem.
Marcus Rashford was fit to start but he struggled to hold up the ball when it came his way and had only 12 touches of the ball before he was substituted in the 59th minute.
Contrast that to Erling Haaland, who brought others into the game and also scored a hat-trick himself. Cristiano Ronaldo was sat with his head in his hands. There appears no way back for him now if he cannot get on the pitch during a defeat like this.