Real Madrid have confirmed that defender Sergio Reguilon suffered a hamstring injury in the 4-2 win at Celta Vigo on Sunday.
The club have not provided a timetable of recovery for the 21-year-old, although it has been suggested among the Spanish media that he could be out for two weeks.
Reports in Spain suggest Nacho is facing two months out and Casemiro will be sidelined for a minimum of three weeks.
The loss of another defender is a blow for interim head coach Santiago Solari, with Marcelo, Raphael Varane, Dani Carvajal and Jesus Vallejo still out of commission.
However, Solari will hope to have Isco back to full fitness after he underwent an operation to treat acute appendicitis in September.
Madrid return to action when they travel to Eibar in LaLiga on November 24.