The Ballon d'Or winner was absent from training on Wednesday because of a calf problem and could miss the big game against Barcelona.
Cristiano Ronaldo
Cristiano Ronaldo raised fears he would miss the first top-flight Clasico clash of the campaign when he was absent from full training on Wednesday.
Real Madrid confirmed the star forward had "worked separately from the group" amid reports he suffered a calf injury in the Club World Cup final triumph over Gremio.
Ronaldo scored the winner in that game and his head-to-head showdown with long-standing Barcelona rival Lionel Messi is the anticipated highlight of Saturday's match at the Santiago Bernabeu.
Madrid are 11 points behind the Camp Nou club, who have not lost in La Liga this season.
Zinedine Zidane's side took the honours when the teams met in the Supercopa, with Ronaldo scoring in the 3-1 first-leg win before being sent off.
He has just four league goals this term, compared to Messi's 14.