Mourinho's weak links exposed by Pep's masterplan
The biggest games are brilliantly instructive. They sort the players who might be handy against Stoke or West Bromwich Albion from those capable of winning you games at nose-bleed altitude against the very best.
Manchester United, who had recruited underwhelmingly since Alex Ferguson retired three years ago, still employ too many standard Premier League players to match the majesty of Manchester City in the first-half on Saturday.
City's confident, incisive passing brought a halt to all those claims that Marouane Fellaini is true Premier League-winning class, or that Daley Blind is an authentic centre-back.
Simply, Pep Guardiola started at a higher base than Jose Mourinho at United, because City's squad were crying out for a manager whose stylistic inclinations were in tune with their individual talents.
The best examples: David Silva and Kevin De Bruyne, who caused havoc in United's central midfield area with their lacerating passes and darting moves.
As this storm went on around him, Fellaini - useful, but ultimately limited - found his natural level again as a player who will serve a purpose in most league games, but not win you one against the new City.
The other sufferer was Paul Pogba, all £89m of him, who was widely criticised for his performance after being marginalised by City's clever play, which flowed right around him.
To Pogba's left, Jesse Lingard was exposed as being either not ready or not good enough to cope at this highest level.
As for Henrikh Mkhitaryan, he joined City's Claudio Bravo in the Patrice Evra club of nightmare Manchester derby debuts.
Being hooked at half-time in a municipal face-off, as Mkhitaryan was, is not sufficient justification to consign a new player to Siberia. He should be given a chance.
However, Mourinho's faintly menacing post-match summary shifted the emphasis away from quality towards attitude, with the implication that some in his starting XI had failed to meet the challenge laid down by City.
Was Mkhitaryan, a £26m buy from Borussia Dortmund, rushed back too soon by Mourinho or simply overwhelmed?
On our side of the pitch, in front of the press box, he seemed to lose confidence quickly and dramatically when his first few touches were less than adhesive and his early forays ran up cul-de-sacs.
United, as we know, were much better after half-time, when Ander Herrera came on with a gladiatorial attitude and Marcus Rashford brought his carefree running to the left-hand side.
Herrera is one of those players often classified as good, but not quite good enough for United. His second-half performance challenged that belief.
Blind, however, was never likely to be the answer as a Premier League centre-half, while Eric Bailly, who has started his United career well, experienced a whole new level of pressure, which he will learn from.
Mourinho will know already that he lacks the general calibre of player Guardiola can call on - which is to City's credit, for purging their squad so decisively.
Eliaquim Mangala, Yaya Toure, Samir Nasri and Martin Demichelis are either out or frozen out (Toure), though Joe Hart would have been an improvement on Bravo.
Those central to Guardiola's plan are plainly inspired by the kind of football he wants them to play.
They like it less, though, when teams come at them hard or high, as United did in the second-half.
Guardiola has an idea and has imposed it. Mourinho is a pragmatist who will proceed game by game.
He has no over-arching vision. He makes mistakes (selecting Lingard and Mkhitaryan), but he corrects them as well, with substitutions and tactical adjustments.
His is a harder job than Guardiola's because he has fewer players of the quality of Silva and De Bruyne.
It is a brutal world that these two Manchester squads now inhabit and Mourinho is already dishing out the put-downs and punishments.
An hour into the derby, Guardiola looked at a Raheem Sterling run come to nothing, turned to his bench and threw up his hands. He had seen enough of the England man and sent on Leroy Sane.
No passengers are allowed in Manchester now, though a few are still managing to cling on.