The catalyst was Mbappé's decision to spend his injury recovery period in Cagliari, Sardinia, with actress Ester Expósito. Cameras from El Chiringuito captured him arriving back in Madrid at 20:48 on Sunday evening, twelve minutes before referee Gil Manzano's opening whistle at the RCDE Stadium.
Mbappé had sustained a semitendinous muscle injury against Real Betis and has been absent for two weeks, with the club's medical services managing his programme.
But the image of a player convalescing abroad while his teammates contested a match with significant title and pride implications caused fury among sections of the Madrid support and unease within the club itself.
Arbeloa opened his post-match press conference by immediately addressing the injury management situation with careful, practised language.
"All the planning for the injured players is supervised and managed by Real Madrid's medical services, who are the ones who control when they have to go to Valdebebas and when they don't."
He then allowed himself a small but loaded addition.
"From there, each player in their free time does what they consider appropriate. And I cannot interfere in that."
When pressed on whether he thought supporters were right to be angry with Mbappé, Arbeloa declined to personalise his answer but extended the discussion in a direction that made the subtext impossible to miss.
"I don't doubt the commitment of any of my players. I think everyone knows how important these matches are for us. When you defend the Real Madrid shirt, when you are in this house, you understand the demands of this club. We are a club where, fortunately, there has never been, is not, and will never be a player bigger than Real Madrid. And I think all my players understand this perfectly."
Asked directly whether Vinicius is more committed than Mbappé — a question that drew a brief, loaded smile — Arbeloa shut down the comparison but expanded into a broader critique of the team's collective effort levels that anyone familiar with his previous press conferences would recognise as directional.
"I don't make comparisons between players. We need everyone. It hurts me when we see that all the other teams run more than us. And not just when we don't have the ball — we also need to focus on this when we do have possession. We need to be a team with much more movement, making far more runs — and it's uncomfortable, because to receive a ball you might have to make ten. We need the commitment of every player to press, defend and attack. Talent alone is not enough."
He then delivered the line that will reverberate across Spanish football this week.
"Real Madrid was not built by players who go onto the pitch wearing tuxedos. It was built by players who finish games with their shirts soaked in sweat, mud, effort, sacrifice and consistency. When the best players in the world understand what Real Madrid is, what its values are, what has made it great — talent and commitment and effort come together. And that is what makes us the best club in the world."
His praise for Vinicius, who scored both goals in a 2-0 win that briefly delayed Barcelona's title party, was as warm as his coded criticism of others was cool.
"He has played a great game again, scoring two great goals, being the leader in the attacking areas. A total threat, very aggressive, very intelligent, very brave. I reserved him because I didn't want to risk him not being available for the Clásico. As I've been saying for months: a fantastic player, a born leader, a teammate that everyone loves, a great person. I'm very proud to have him as my player. I'm incredibly lucky."
The contrast with his Mbappé response was stark and unmistakable. Arbeloa was asked whether the French forward would be fit for next Sunday's Clásico at the Camp Nou and whether Dani Carvajal, who sustained a toe fracture this week, had any chance of featuring before the season ends.
"We'll see how Mbappé progresses this week. After last week's tests it seemed like it might be a bit longer, but we'll see how he evolves. As for Carvajal — I still have high hopes that he can play before the end of the season, recover and finish the season on the pitch. That is what Dani Carvajal deserves."
Mbappé has scored 41 goals and provided six assists this season. His numbers have not been the problem. The question being asked loudly in Madrid tonight is whether the man who produces them shares the values that the club insists are non-negotiable.