Forget the league title for a moment. This was an object lesson in why Manchester City are going to struggle to keep Liverpool’s players off the scoresheet when the Champions League quarter‑final resumes on Tuesday.

At the moment City require four goals to go through – should Liverpool score they will need at least five – and allowing a feeble Manchester United back into this game from a position where they should have been dead and buried will have done nothing for their confidence.

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Even with a weakened side City were so far ahead in the first half it was embarrassing. United were doing nothing to stop David Silva finding space to run the game and must have been grateful to arrive at half-time only a couple of goals in arrears. The visitors, a bit like City at Anfield in midweek, looked as if they could play all day without scoring – until two goals in two minutes from Paul Pogba wiped the smiles from the faces of the champions-elect.

Both were created by Alexis Sánchez, both involved misjudgments from Nicolás Otamendi. The defender strayed out of position to try to dispossess Sánchez by the touchline, only to be left spectating when the Chilean executed a neat turn and sent the ball into the box. Moments later Sánchez found a pocket of space on the opposite flank and sent in a diagonal cross for Pogba, who got goalside of Otamendi and placed a header just inside Ederson’s right-hand upright.

Either this was the most cunning of gameplans from José Mourinho, using the first half to lull City into a false sense of security, or the home side had once again shown their propensity to crack under very little pressure. If Pep Guardiola’s side are this fragile playing at home against neighbours who spent the first half seemingly content to witness a coronation, they are not going to last much longer when the going gets tough in the Champions League.

To say the first half gave no hint of what was to follow would be to risk the understatement of the season. After the chasing they were given on Merseyside, City found themselves invited to relax in Manchester, largely due to the fact that United these days are far too languid to resemble Liverpool even remotely.

The reason the best team in the country swept to a two-goal lead with their best players left on the bench was because United allowed them to play their normal game. Liverpool had denied them that luxury in the Champions League but here City were able to build from the back, cross the halfway line with impunity and patiently attempt to pick a way through the retreating red shirts.

With players such as David Silva and Raheem Sterling in their side City were always likely to be able to unlock a defence that kept funnelling backwards when the ball was lost, and the opening goal was a case in point. While it might have come from a set piece, Vincent Kompany planting the firmest of headers past David de Gea from Leroy Sané’s corner, the corner itself came from City winning the ball back in midfield and working it quickly towards goal. Sterling lost possession near the centre circle but tenaciously worked to regain it and from then on United could only counter quick passing and movement from Silva and Sané by putting the ball out of play.

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Until that point there had been some doubt as to whether Guardiola might have to call upon Kevin De Bruyne or Kyle Walker to inject creativity and pace, though United did not change their approach on going behind and the home side were two up within five minutes, Ilkay Gündogan ultimately making the most of a poor clearance from De Gea that merely invited Sané to set up another attack. It is no exaggeration to say City could have been five goals up by the interval and that Sterling could have had a first-half hat-trick, though as the Etihad knows only too well the England winger is not that reliable a finisher.

It was the usual story when Sterling missed his first big opportunity. The pass from Silva that put him clear was just too good, it gave him too much time and, with Antonio Valencia thundering in on his left, Sterling dithered a moment too long and scooped his shot wastefully over the bar. The next chance also came from Silva and once again Sterling could not keep his shot on target. Eventually, as the interval neared, he did find space to measure a shot that required a save but his weakly hit effort was little more than a back pass for De Gea to gather.

With United showing zero inclination to fight back City should have been so far ahead at half-time they could have paraded the Premier League trophy as half-time entertainment.

Pogba and Chris Smalling changed that perception utterly in the second half, an improbable comeback that means City have lost back-to-back games precisely when they were meant to be sweeping all before them. If Liverpool were even thinking about parking the bus on Tuesday this result should make them reconsider. Their opponents have just conceded six goals in 180 minutes.