The last time Jurgen Klopp’s Liverpool went to Etihad Stadium for their Premier League fixture, things started to go wrong midway through the first half.
One early symptom of the defensive uncertainty that would lead to Liverpool's heaviest defeat in Manchester for 70 years was Trent Alexander-Arnold's hesitation making a clearance. As he procrastinated, Kevin de Bruyne pounced, and the Liverpool full-back, realising his error had put his team into trouble, brought the Belgian down and was booked.
City went 1-0 up five minutes later. Thirteen minutes after Sergio Aguero's opening goal, Liverpool were down to 10 men following Sadio Mane's dismissal for dangerous play.
Goals then seeped past a diminishing Liverpool at steadily staggered intervals. Gabriel Jesus, 20, scored twice and Leroy Sane, 21, adding numbers four and five. Alexander-Arnold, still a teenager back in September 2017, looked on, apparently unready to share the same elevated stage as Jesus and Sane.
Since then, Alexander-Arnold has played in a European Cup final. He has been on the winning side twice against City, who have lost three of their four contests with Liverpool since the 5-0 thrashing. He has also been part of England's World Cup campaign that ran all to the way to the semi-finals.
He is entitled to reflect on how much he has developed, and perhaps on how much his manager has wised up, too.
Much has altered since the September day when City established firmly that they were the likeliest 2017/18 English champions, as they a Liverpool whose first six positions in the XI were occupied by Simon Mignolet, Alexander-Arnold, Joel Matip, Ragnar Klavan, Alberto Moreno, and, anchoring midfield - rather loosely, as it turned out - Emre Can.
Fast forward to the Liverpool who now lead the Premier League, seven points ahead of the City whom they visit on Thursday night. The back end of Klopp’s preferred team looks distinct in all but Alexander-Arnold’s position.
Can has joined Juventus and Klavan moved to Cagliari. Mignolet, Matip and Moreno have all been overtaken in the squad hierarchy.
The evolution of fortress Liverpool is startling. After their first 21 Premier League matches in 2017/18, Klopp’s defence had conceded 24 goals; they go into matchday 21 of 2018/19 - the D-day against City - having let in just eight.
The clearest transformations are in goal and in the centre of defence, where a combined €150 million (Dh630m) was invested in Alisson Becker and Virgil van Dijk.
The latter provided conspicuous leadership and expert marshalling at the heart of the back four and the former the sort of confidence, as a goalkeeper-sweeper, that his defenders take their lead from.
Goalkeeping had become Liverpool's problem position last season, culminating in the dreadful experience of Loris Karius - who unconvincingly contested the role with Mignolet - in the Uefa Champions League final defeat to Real Madrid.
The influence of Van Dijk can hardly be overestimated. Since he head in the winning goal almost exactly a year ago, on his Liverpool debut in an FA Cup derby against Everton, he has made the tag of the world's most expensive defender look less like a burden than a deserving badge of honour.
Alisson - bold, assertive and Brazil’s first choice - has taken to his responsibilities with similar gumption.
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"I was always very clear Alisson would be a very influential player, but I honestly did not see he would be this good," Klopp beamed about his goalkeeper after the 5-1 win over Arsenal last weekend.
Klopp counts two errors so far in Alisson's first six months in English football, and says he would have been ready to pay even more than the €70m Liverpool agreed with Roma to sign him.
Less heralded than "Virg" and "Ali", as the Liverpool manager calls the two men who have most obviously upgraded his side, is Andy Robertson, signed for a modest €9m from Hull City 18 months ago.
There is perhaps no footballer that City - without first-choice left-back Benjamin Mendy because of injury - would more envy from the Liverpool squad right now than Robertson, who is positionally disciplined and an excellent crosser.
Klopp will note than so far this season, his full-backs, Alexander-Arnold and Robertson are bettered in the number of assists they have supplied only by Mohamed Salah.
Unbeaten in the league, Klopp’s one concern about his transformed defence is the absence, with injury, of Joe Gomez, who was used at right-back, rather than his usual role alongside Van Dijk, in the last meeting with City, the goalless draw at Anfield in October.
Dejan Lovren will likely play at centre-half, with Alexander-Arnold manning the right flank, that 5-0 rout at the Etihad a long way back in his rear-view mirror.
