The Portuguese superstar controversially said after the derby loss that "if we were all at my level, maybe we would be leaders," as well as questioning the quality of squad players Lucas Vazquez, Jese and Mateo Kovacic, per Marca (via Sky Sports' Pete Hall).
In an attempt to reconcile with the rest of the squad, Ronaldo, 31, reportedly "felt obliged to apologise via WhatsApp to some of the team-mates he upset," per El Mundo (via Sport's Francesc J. Gimeno).
The report added that Ronaldo was also planning to explain his comments to the squad before training on Monday.
He had already attempted to clarify his criticism in the media by explaining his comments, per Marca (via Hall): "I was referring to the physical level, not level of play. I am not better than any of my team-mates."
However, Bleacher Report's Dean Jones said that he thought Ronaldo was right to call out some of his team-mates, who did not perform to their best against Atleti:
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The 1-0 defeat—Antoine Griezmann netted the only goal in the 53rd minute—means Real are now 12 points back from La Liga leaders Barcelona and effectively out of the title race.
They badly missed injured duo Gareth Bale and Marcelo on Saturday, and Karim Benzema being forced off at half-time was also a big blow.
The loss ended an eight-match unbeaten run under manager Zinedine Zidane since he succeeded Rafael Benitez in January.
The Frenchman ceded the Spanish top-flight title after the defeat, per Bleacher Report UK:
But Los Blancos are still going strong in the UEFA Champions League and will be desperate to go all the way in Europe's premier club competition.
That may be a challenge, though, if Zidane does not have his full squad to call upon, or if his star player continues causing rifts in the dressing room.
Zidane and Ronaldo must both hope the Portugal captain's apologies are accepted or else they could have a bigger problem on their hands at Madrid in the long-run.