It was a game where, if you believe the Spanish media, Zinedine Zidane's job was on the line.
Sergio Ramos, who had been injured and missed the 3-2 capitulation to Shakhtar Donetsk in midweek, returned in supreme style, doing everything right defensively and slotting home the crucial penalty kick which gave his side an advantage to defend.
And Madrid very much relished playing on the counter; in virtually every outing this season they have been asked to break down a low block and, in the main, they don't enjoy it. Zidane hasn't found a system that can unlock stubborn teams no matter the formation or personnel.
But this was an occasion where Zidane's tried-and-tested individuals stepped up, while Barcelona's largely did not. The likes of Ramos, Thibaut Courtois and Fede Valverde performed with distinction and played the occasion brilliantly. The latter scored a superb opener while Courtois is big at the big moments, making a wondrous save from Lionel Messi in the first half.
And while Koeman was bold in his team selection, it ultimately didn't pay off. Pedri was given a start and a sign of faith from the Dutchman but the 17-year-old completed just 79% of his 39 passes and didn't create any opportunities in the game. Indeed, it was bizarre that he lasted 81 minutes of an occasion that was a little too big for him.
Likewise, Sergio Busquets has been a magnificent servant to Barcelona but his drop-off in pace is more noticeable than ever and Madrid clearly targeted him, harrying around him and exposing him as the weak link, and he was found wanting.
His performance also affected Frenkie De Jong alongside him and it's clear that Miralem Pjanic has to be play more often as part of the two. Busquets' decline has been happening for 12 months or more, but this could be the game that tipped the scales.
And Koeman also gave Philippe Coutinho far longer then he deserved. As well as missing a glaring header from close range at 1-1, he made the wrong decision too many times and he remains in thrall to Messi. The Brazilian has been better this season but this wasn't the best display of his talents and it shows Barca lack the correct balance against quality opposition in the front six positions.
But what will be most galling for Cules is that Madrid, after the week they have had, were there for the taking. They were vulnerable and Barca could have delivered a crushing blow. Instead, they have been given life again.
Koeman was brave but it didn't pay off and he waited far too long to change things. He is working under a difficult environment, clearly, given the presidential politics at play, but he still had talent on the bench that he failed to utilise. £250m worth of talent - Antoine Griezmann, Ousmane Dembele, Francisco Trincao and Martin Braithwaite - were deployed when it was effectively too late.
And so Madrid live again. A team that, with the legendary Ramos, is unrivalled in terms of game management, understanding what to do and when. Barcelona have lost that shrewdness and it showed here. Koeman has work to do to find the correct blend of youth and knowhow to give Los Blancos a title race.