The times they are a changing back in Manchester. For the first time in ages United fans may have more cause for optimism about the future than their City counterparts.
Scott McTominay's spectacular injury-time clincher was a neat microcosm of the game as a whole. Ederson's careless throw was just one example of the colossal carelessness which characterised City's display.
McTominay's first-time finish, on the other hand, encapsulated the confidence and enthusiasm of a team on the way up. Once he might have taken a couple of touches, looked for a teammate or even set off for the corner flag. But the old confidence is returning at Old Trafford. There's no need to get carried away and hail Ole Gunnar Solskjaer as the saviour. Failure to secure a Champions League place would still represent underachievement in a mediocre season. But ten matches unbeaten suggests a top-four spot or Europa League victory is not beyond United.
Three United wins out of four in this season's Manchester derbies indicates a shift in the city's balance of power. City can not explain away yesterday's defeat as they did the previous two. This was neither an ambush nor a second-leg match where City's qualification was virtually assured beforehand. Pep Guardiola's team were amply forewarned but fell short all the same. United were just better.
Guardiola's future at the club and maybe even the entire City project now rests on the Champions League. It is the basket into which all the club's eggs have been placed. Should they fail to capture it, defeats like this one and the careless performances which facilitated them will look even worse in retrospect.
The strategy which City have forced on themselves is a high-risk one. They should get past Real Madrid but Bayern Munich, Liverpool, Barcelona, Paris St-Germain and Juventus could still bar the way. One thing those teams have in common is that they are still contending for league titles.
City are not. They're not bidding to emulate the all-conquering Manchester United of 1999 or Barcelona of 2009 and 2015. They are chasing an achievement comparable to Liverpool of 2005 and Chelsea of 2012, who compensated for domestic underachievement by getting lucky in Europe. This is where the world's biggest-spending team are at now.
This latest defeat furnished the clearest proof yet that, in the domestic arena at least, City have entered a period of decadence. The overwhelming possession statistics once proudly flourished as proof of superiority don't mean much anymore.
At Old Trafford the visitors had 69 per cent possession in the first half and just one shot at goal. At times they seemed to be passing the ball around just for the sake of it rather than with any end in mind. This lassitude was combined with a sloppiness which reached its height during a period early in the second half when you could practically see City crumbling in front of your eyes.
Forty-eight minutes: Ederson lets a backpass roll under his foot and just about prevents Anthony Martial from scoring; 52 minutes: Raheem Sterling lets an easy pass roll under his foot and out over the touchline; 53 minutes: A promising breakaway comes to nothing when Sterling plays the ball way behind Sergio Aguero; 54 minutes: Fernandinho is easily outmuscled by Anthony Martial; 57 minutes: Joao Cancelo gives the ball away to Brandon Williams and commits a spiteful foul out of exasperation.
This collective nervous breakdown came to an end when Guardiola brought on Riyad Mahrez and Gabriel Jesus in the 58th minute. City improved after that but for all the possession, only two clear chances were created, both in the 74th minute when, after Sterling failed to convert a Mahrez cross six yards out, Jesus forced a save from David De Gea.
It was United who looked more likely to score the game's second goal, thanks to Daniel James' one-man exhibition of the counter-attacker's art. One run produced a cross marginally too high for Martial, one ended with a shot well saved by Ederson and another came to nothing because Bruno Fernandes strayed offside. But the purpose and direction in those forays put City to shame.
The popular cliché is that City are being undone by defensive weakness. But there's more to this season's misfires than that.
City may still be the Premier League's leading scorers but they have found goals difficult to come by against quality opposition. Their build-up can become pedestrian, particularly in the absence of Kevin De Bruyne, on whom they've come to depend to a ludicrous extent. This season the great Belgian has provided more assists than David Silva, Bernardo Silva, Raheem Sterling, Rodri and Ilkay Gundogan put together.
Last season's intensity has been missing. When City went 2-0 up against Aston Villa in the Carabao Cup final you expected something similar to last year's six-goal FA Cup final trouncing of Watford. Instead only the woodwork prevented the game being brought to extra-time.
City seem to be having problems maintaining interest in domestic competition. The enervated end-of-era quality of their football makes you wonder if Guardiola has taken them as far as he can. He has never been a man for the long haul. The club's owners might be thinking in terms of a successor.
But who? Nuno Santo? Brendan Rodgers? Julian Nagelsmann? Massimiliano Allegri?
Probably none of the above. Should Guardiola fail to propel City to Champions League nirvana, his most likely replacement is a man who may already have done some research into life in Lancashire.
Mauricio Pochettino could be coming to Manchester after all.