Ederson has been a boon for City in the way he joins in the outfield play, and distributes the ball.
But there has long been a suspicion that his actual goalkeeping can be very ordinary - and it dipped below that as he allowed a tame Anthony Martial effort to sneak inside his near post for the only goal.
Guardiola has a reputation for playing his strongest team – or something close to it – when you least expect it.
But here he did the reverse, refusing to risk Kevin De Bruyne for the derby, and rotating both his full backs, with Kyle Walker and Benjamin Mendy both on the bench.
Not playing your strongest team against United, especially at Old Trafford, would – in the past – have been an act of self-harm, both in terms of the manager’s standing with the fans, and, more than likely, in terms of the result.
But Guardiola’s remarkable record, and his ability to field a team which was good enough to win the game in front of him, has earned the trust of the City fans.
It looked like he had done it again in the opening 20 minutes, as City’s high press confounded the Reds and forced them into hurried football
And when Sergio Aguero snapped at Luke Shaw and stole the ball, they should have taken the lead.
Aguero was forced wide but managed to keep the attack going, and when Phil Foden found Raheem Sterling in space, a player of his reputation should have done more with his curling shot than place it at a comfortable height for David De Gea.
The Blues were in total control, hemming United into their own half as Fernandinho, Rodri and Ilkay Gundogan turned the screws.
Aguero nutmegged Maguire but saw his shot taken off his toe – that would have been an iconic derby moment had he finished it, but it has been the story of City’s season that they have got into winning positions and then flunked it.
The early intensity flagged, however, and United took advantage.
They benefited from a poor Mike Dean decision as Gundogan brushed past Bruno Fernandes with minimal contact, but the Portuguese star performed a double twist that somehow convinced the ref it was a free kick.
Fernandes chipped that undeserved free kick for Anthony Martial as the City defence went to sleep, and his tame effort somehow sneaked under Ederson’s outstretched arm and inside his near post.
Suddenly United found their belief, and the Blues were exhibiting the defensive uncertainties which have cost them this season.
The video assistant seemed to have taken a day off, but that benefited City when the perpetually clumsy Nicolas Otamendi clattered into Fred in the area.
Dean pre-empted himself by brandishing a yellow card at Fred for a non-existent dive, while VAR took a snooze.
Otamendi had one of his better games in the 3-1 win at Old Trafford in the Carabao Cup semi-final, but he returned to his normal setting of “liability” in this one.
Much had been made of Foden’s first start in a derby, but he was given a sacrificial role, hugging the touchline to give others space inside, giving him little chance to make an impact.
He tested De Gea to the full with a driven shot from 25 yards, early in the second half, but the Spanish keeper leaped to tip it over the bar.
The teenager's inclusion in the team was a seminal moment for him, but he had been marginal enough to be twitchy when Guardiola decided to make a double substitution on the hour mark.
But Foden remained, and not only that was switched into a central role as Riyad Mahrez went on to fill the right-wing berth.
The Algerian created a great chance for Sterling, who has not scored against United in 20 appearances, skidding a cross, but it was fractionally ahead of the England man and his toe could only divert it away from goal.
Sub Gabriel Jesus retrieved it and forced a diving save from De Gea but the BLues rarely troubled a packed United defence.
And in added time, Ederson compounded his nightmare with a slack throw straight to Scott McTominay, who promptly fired it back into the unguarded net to secure the points.