Sergio Ramos is a major doubt for Real Madrid's Champions League trip to Apoel Nicosia after he suffered a broken nose in the Madrid derby on 18 November.
Ramos was inadvertently kicked in the face by Atletico defender Lucas Hernandez when attempting a header in the first half at the Wanda Metropolitano and was initially able to continue playing after Real physios plugged up his nose with cotton wool.
However, the Spain captain was withdrawn from action at the half-time break, with Nacho taking his place in the second half.
Real coach Zinedine Zidane confirmed that Ramos had suffered a fracture of the nasal septum and stated he was unsure how long the centre-back would be out of action due to the injury.
"I did not see [Hernandez's challenge on Ramos], and will not comment," Zidane was quoted as saying by ESPN.
"The referee did his job, that's it. It is a fracture, for Sergio. I believe it's broken. I don't know how long he'll be out for."
Emilio Butragueno, Real Madrid's director of institutional relations, said it was a "bad sign" that Ramos was unable to continue playing after half time.
"Ramos is going to have tests but if he has left the game it is a bad sign," he was quoted as saying by the Sun. "We have to be careful and see what the doctors tell us."
The stalemate at the Wanda Metropolitano means Real are now 10 points behind La Liga leaders Barcelona, who beat Leganes 3-0 on 18 November.
No team has recovered from a 10-point deficit to win the La Liga title, but Zidane said his team would not give up on their title pursuit.
"There are still many games to go, many points to play for," the Frenchman said. "Ten points are a lot, yes, but for sure this will change.
"Barca will not always pick up points, and we must be there waiting. Football can change quickly, and we will be there fighting until the end, for sure."