Real Madrid confirmed Sunday that left-back Ferland Mendy had suffered a thigh injury and will be unable to face Atalanta on Tuesday in the Champions League.
Mendy came off hurt in the second half of Madrid's 3-0 La Liga on over Girona on Saturday.
"Following tests carried out on Ferland Mendy today by Real Madrid's medical department, the player has been diagnosed with a muscular injury to his right quadriceps," said Los Blancos in a statement.
Madrid did not state how long the French international would be unavailable, but Spanish media reported he is likely to be sidelined for several weeks.
Madrid are fighting for Champions League survival after three defeats in their opening five games.
Mendy is also set to miss La Liga games against Rayo Vallecano and Sevilla, as well as the Intercontinental Cup final on December 18.
Jude Bellingham also sustained a knock in the clash with Girona but coach Carlo Ancelotti said he would be "available" for the Atalanta game.
Brazilian attackers Vinicius Junior and Rodrygo Goes are expected to return for that match, but Dani Carvajal, Eder Militao, Eduardo Camavinga and David Alaba are still out injured.