Ronaldo under fitness cloud ahead of El Clasico
The 32-year-old is yet to join the main group since reportedly picking up a calf problem in the Club World Cup final victory over Gremio.
In a statement, the club explained Ronaldo "trained alone out on the pitch, both with and without the ball".
Losing him for Saturday's crucial clash would be a big blow to Los Blancos' chances of cutting the gap to Barcelona back to eight points.
While he has only scored four times in LaLiga this term, Ronaldo is just one shy of Alfredo Di Stefano's all-time Madrid record of 18 goals against Barcelona.
The hard work continued at #RMCity this morning.
Two days to go until #RMClasico!
: https://t.co/7RWaaZrmM8 pic.twitter.com/hKu3xOOBQx
— Real Madrid C.F. (@realmadriden) December 21, 2017