At various points during the early stages of this Premier League season, Manchester City and Manchester United had their defensive capabilities called into question by television pundits.
As soon as the opening weekend 3-1 defeat to Crystal Palace, in fact, Gary Neville emptied a bucket of cold water upon transfer speculation surrounding Ole Gunnar Solskjaer's squad.
Speaking on Sky Sports, the former England and United defender gave a damning assessment of Harry Maguire and Victor Lindelof's centre-back capabilities
"You can talk about Jadon Sancho all you like, but until they get a centre-back in that can run and defend one on one, you're never going to win the league," he said.
A weekend later, City were hammered 5-2 at home to Leicester City – a result that immediately preceded the club-record arrival of Ruben Dias from Benfica.
The Portugal international's debut came in a harum-scarum 1-1 draw at Leeds United, after which beIN Sports' anchor Richard Keys offered a more radical solution to Pep Guardiola.
"It can't be that hard, he could go and watch Roy Hodgson work. Or he could bring Sam Allardyce in on a temporary basis."
Things have changed a little since then. City head into Sunday's top-of-the-table clash 14 points clear of United in second and aiming to extend a 21-match winning run across all competitions.
Central to that, and untouched by the hand of Allardyce, is a water-tight defence and the supreme alliance formed by Dias and a rejuvenated John Stones.
Silk and steel
In the 14 Premier League games Dias and Stones have started in tandem, City boast 13 wins and a draw, keeping 11 clean sheets
Goals for each man in last weekend's 2-1 win over West Ham mean they are responsible for four goals at the other end during these matches – one more than the three they have conceded overall.
In his debut Premier League season, Dias boasts a win percentage of 75 per cent thanks to 18 victories from 24 matches.
Stones' own win rate leaps to a frankly absurd 94 per cent, finishing on the winning side in 15 of his 16 Premier League outings in 2020-21. Guardiola's men have only conceded four times with him on the pitch.
Though Keys opined that Guardiola might have been sacked for his "reckless" spending before settling upon Dias, acclaim for the Catalan tactician and his first-choice central defenders has otherwise been close to universal.
Over on the red side of Manchester, however, Neville still isn't happy.
Drawing blanks and protecting keepers
Despite being the centre-back pairing in each of United's five 0-0 Premier League draws in "big six" encounters this season, last month Neville described Lindelof and Maguire as a "problem" to one another on account of their individual attributes.
Demanding defensive changes for a team who have just run up three consecutive goalless draws in all competitions might, on the face of it, look like a fairly off-target conclusion.
However, outside of those matches that were as high on anticipation as they proved low on goals, another picture emerges.
In 20 Premier League games together, Lindelof and Maguire have seven clean sheets this term – only two outside of big-six encounters. United have won half of those games, drawing eight and losing two, leaving them way down on City's 92.9 per cent success rate when Dias and Stones start together.
Overall, United have conceded nearly double the amount City have let in across the Premier League season, with 32 set against 17.
Expected goals on target (xGOT) numbers collected by Opta – figures that illustrate the overall quality of chances faced - show City's first-choice goalkeeper Ederson is better protected by his defence than United number one David de Gea.
Ederson has conceded 15 times and has an xGOT of 18.61, meaning he has conceded fewer goals than expected, something that cannot be said for De Gea, who will sit out the weekend game after returning home for the birth of his child.
An xGOT figure of 25.9 is outstripped by the 27 the Spain international has let in. Although his negative goals prevented score of 1.1 does Maguire, Lindelof and those others in front of him a slight disservice, he is being given more taxing work to get through than Ederson.
Building from the back
It is not the case that Stones and Dias are flying into numerous last-ditch challenges to spare their goalkeeper, much as they appear to relish that work when it arrives.
A chunk of their value to Guardiola stems from how they are each able to get City swiftly back on the front foot.
Every 90 minutes, Dias averages 16.3 progressive carries, with Stones on 14.7. On this metric, despite his noted ability with the ball at his feet, Maguire tallies 11.8, with Lindelof down on 9.8.
Building play from the back is another non-negotiable for Guardiola's defensive players and City's 161 build-up attacks – open play sequences featuring 10 or more passes that end in a shot or a touch in the opposition penalty area - are the most in the Premier League this season.
Despite the apparent risk they run of playing themselves into trouble, 120 high turnovers against and 14 shot-ending high turnovers are once again the best results in the division.
By contrast, United have conceded 35 shots from high turnovers – the second-worst in the league – from 205 such instances. Solskjaer's men have attempted 91 build-up attacks, suggesting his team create too many problems for themselves.
Stones and Dias have managed their shutouts compilation despite City having the highest defensive line in the Premier League, starting attacks an average of 45.2 metres from their own goal. United's 42.9m is third behind Liverpool, although the lack of pace Neville has highlighted in Solskjaer's first-choice pairing might be encouraging a measure of caution.
Even if Maguire and Lindelof are able to wheel out their effective spoiling act to thwart the City juggernaut this weekend, Neville is right that the bigger picture needs better solutions at his old club. Although, probably nothing as outlandish as suggesting Sam Allardyce as a specialist defensive coach.