Sergio Ramos has a calf injury and is expected to miss Real Madrid's first La Liga match after the winter break.
The Madrid captain was absent from Zinedine Zidane's squad for Thursday's 3-0 win at Numancia in the first leg of the Copa del Rey last 16, although fellow big-name players such as Cristiano Ronaldo were also rested.
And Madrid confirmed in a brief statement on Friday that Ramos would be out of action for an unspecific period due to injury.
"Following tests carried out on Sergio Ramos by the Sanitas Real Madid Medical Services, he has been diagnosed with a muscular injury in the soleus of his left leg," Madrid's statement said.
"His recovery will continue to be assessed."
Madrid travel to Celta Vigo on Sunday, having lost their last La Liga fixture before the winter break 3-0 at home to Barcelona, resulting in Zidane's men slipping 14 points behind their rivals.