The Portugal international appears to be a doubt for the contest at the Santiago Bernabeu, and Los Blancos posted a bulletin stating he "trained alone and without the ball," as relayed by Football Espana.
As noted in the report, Ronaldo is said to have picked up a calf issue last weekend during the Club World Cup final against Gremio.
On Wednesday, Madrid said "Ronaldo trained apart from the rest of the group" but nothing more. Per the report, the Los Blancos talisman was anticipated to start alongside Gareth Bale and Karim Benzema in the clash.
If Ronaldo were to miss the weekend clash, it would add to what has been a frustrating campaign, by his own high standards at least.
While he's performed well in the UEFA Champions League and in the Club World Cup, in La Liga, the forward has netted just four goals in 11 appearances. As noted by Spanish football writer Colin Millar, he's still been prolific overall:
Madrid fans would have been encouraged by the recent form of Ronaldo. In his last La Liga outing he scored twice against Sevilla, and he netted in both matches at the Club World Cup.
On the brink of such a big game, manager Zinedine Zidane will be aware he can rely on Ronaldo. In recent years, the Portuguese has often risen to the occasion on the grandest stage, whether that be in finals or Clasico fixtures.
As we can see courtesy of OptaJose, Ronaldo tends to save his best for games against Barcelona and is closing in on a club milestone:
Madrid need to win this fixture, as they trail a rampant Barca by 11 points in La Liga having played a game less.
Defeat to their biggest rivals would not only sting, it would surely also spell the end of any hopes Madrid have of defending their title, while a draw would still leave Los Blancos with a mountain to climb.
All those donning the white kit on Saturday will feel much more confident of picking up those vital three points if Ronaldo is alongside them on the pitch.