Real Madrid's defeat to Atletico Madrid was no ordinary derby wound. It once again laid bare a tactical problem that has forced its way to the heart of the team, and the headline writes itself: the overlap between Kylian Mbappe and Vinicius Junior.
Los Blancos went down 2-1 at the Wanda Metropolitano, a result that left plenty of question marks over their display. Their two attacking jewels, Vinicius and Mbappe, both vanished, struggling badly to find space and manufacture danger.
According to the Spanish newspaper "Marca", the players' movement map during the derby laid the same problem bare. Vinicius operated mainly on the left wing, drifting inside time and again, while Mbappe started as an out-and-out striker in Mourinho's 4-2-3-1, only to gravitate heavily towards the left flank himself.
You can discuss the topics and news in greater depth in the "Kooora" forums
The upshot was obvious: "the presence of both players in almost the same areas, which reduced the effectiveness of each of them and weakened Real Madrid's attacking variety".
This is no product of one derby. It stretches right back to Mbappe's arrival at Real Madrid, where the brilliance of one star has repeatedly come at the expense of the other.
Marca report that the same scenario has now played out under four different coaches: Carlo Ancelotti, Xabi Alonso, Arbeloa and Jose Mourinho. None has landed on a fix that gets the best from both men.
Disastrous numbers in the derby
Beyond the movement map, Real Madrid struggled alarmingly going forward against Atletico.
They ended the game with an average of just 0.32 expected goals, their lowest figure in a La Liga match since February 2015, according to Opta data.
Mbappe managed only one shot across the 90 minutes. Vinicius fared little better with two attempts before Mourinho hauled him off in the 54th minute.
You can discuss the topics and news in greater depth in the "Kooora" forums
The stakes only rise with the renewed chatter about Mbappe's role with France. Zinedine Zidane, the national team's new manager, has hinted at his admiration for the striker's qualities when he plays on the left wing.
French reports, including "RMC Sport", state that Zidane has already contacted Mbappe and told him he will use him in that position for France.
So Real Madrid find themselves grappling with an old yet renewed dilemma: "how can Mbappe and Vinicius be combined without the presence of one becoming an obstacle to the other?"
With the derby lost and the attacking numbers plummeting, the ball now sits firmly in Mourinho's court. He must find a tactical formula that lets the pair roam freely without one cancelling out the other.