With many Spanish internationals at Barcelona and Real, tensions could creep into the national team.
As Real Madrid and Barcelona player relations simmer, La Roja could suffer
Television viewers across the Spanish-speaking world who tuned in last Wednesday night to the state broadcaster TVE's coverage of the Copa del Rey final kept hearing the same phrase during the commentary. It was "companeros de la seleccion", meaning "teammates in the national team".
It would be uttered whenever Sergio Ramos clattered into Andres Iniesta, or if Xavi intercepted a pass delivered by Xabi Alonso.
The fact they are Spain colleagues would doubtless have been pointed out if Real Madrid's Raul Albiol had fouled Barcelona's David Villa, although because that scenario had already been played out in the previous league meeting between the two clubs, Albiol was already suspended for the cup final.
Albiol, the central defender, is available again for this evening's third of four matches between Spain's super-heavyweights in the space of 18 days. That raises to 13 the number of players potentially on the pitch tonight who together won the last World Cup as colleagues for La Roja, the Spanish national team.
The "national team colleagues" reference will be made again and again in commentary. It gets delivered with pride when the seven Barcelona footballers who all featured in Spain's starting line-up last July 11 in Soweto, South Africa - to grasp, for the first time in the country's history, the World Cup - pass to one another, as they can do with such sweet efficiency.
But repeatedly noting that players on opposite sides of club football's most famous rivalry are joined by their success with Spain also betrays an anxiety.
The 210 minutes Real and Barca have spent so intensely in each other's company over the past 11 days has been punctured by face-to-face confrontations that risk making dinner times at the national squad's headquarters come June, when Spain next play, a little awkward.
Nor is Albiol the first Real player to have been sent off in a clasico this season. Ramos walked near the end of Madrid's 5-0 defeat at Camp Nou last November. Or rather he stormed off after receiving a red card for a hot-tempered, ill-timed challenge on Lionel Messi.
The foul agitated all the nearby Barcelona players, most of them Spain internationals; on his way off the pitch, Ramos lunged at Carles Puyol, the Barca captain, grabbing him by the throat.
That's the same Puyol with whom Ramos shares responsibility, successfully, in the back four of Spain. It's the same Puyol with whom Ramos divides up much of the extra-curricular activities when La Roja are away at tournaments, when 23 young men need to find ways to let down their hair together, to combat the tedium of hotel life. Puyol organises the sweepstakes, Ramos takes charge of the music they listen to on buses and elsewhere.
By most accounts, Puyol and Ramos get on well when with La Roja. So, apparently, do Real's Iker Casillas and Barcelona's Gerard Pique, although Casillas, who is Spain's and Real's captain, felt it necessary to chide Pique via the medium of Spanish radio last week.
It had been reported that after the league draw at the Bernabeu, the bumptious Pique, 24, had made provocative comments, about the distinctions between the region of Catalonia - of which the city of Barcelona is the capital - and Spain, alluding to Catalan claims for independence. Pique denied it; Casillas retorted: "He knows what he said and so do my colleagues."
Even the milder Spanish individuals, such as Real's Alvaro Arbeloa and Xabi Alonso, bristle against the more pugnacious Barcelona men, such as Sergio Busquets. Arbeloa made a horrible studs-up challenge on Villa in the cup final.
Beneath all this duelling simmers another tension that impacts on the holders of the World Cup and European Championship. Spain's unprecedented success has been based, dogmatically and determinedly, on passing football, on giving priority not to muscle but to mastery of the ball.
In that, Spain mimic Barcelona. La Roja borrow more heavily for Barcelona's crop of mainly Catalan players to achieve their smooth routines and rhythms.
Yet the football with which Real have in the past 11 days drawn with and beaten Barcelona is of a different hue. It has been brutal, physical; it has forgone the principal of possession and intricate passing.
Loyalists to the national team fear not only that the World Cup heroes might fall out with each other over the course of these Champions League clashes, but that Spain itself might start doubting what is the best way to win matches, and gain trophies.