Real Madrid have confirmed that Ferland Mendy has suffered a muscular issue, with the Frenchman set for another spell on the treatment table.
Mendy, 30, has only made one appearance for the capital giants this season - playing the full 90 minutes against Olympiacos in the Champions League on November 26.
The France international was absent in the early months of the campaign due to a thigh issue, only returning to the bench at the end of October, but he is now facing more time out, with reports in Spain suggesting that the defender could be absent for the remainder of 2025.
"Following tests carried out today on our player Ferland Mendy by the Real Madrid Medical Services, he has been diagnosed with a muscle injury in the biceps femoris of his right leg. His recovery will be monitored," read a club statement.
Mendy is one of four Real Madrid players out of action, with Dani Carvajal (knee), Dean Huijsen (muscle) and David Alaba (calf) on the sidelines.
Real Madrid confirm fresh Mendy injury blow
The left-back made the move to Bernabeu from Lyon in 2019, and he has represented the capital giants on 202 occasions, scoring six goals and registering nine assists.
There is uncertainty when it comes to Mendy's future, though, as he has lost the starting role at left-back to summer arrival Alvaro Carreras.
Real Madrid have five matches before the end of 2025, facing Athletic Bilbao, Celta Vigo, Alaves and Sevilla in La Liga, in addition to a Champions League clash with Manchester City, and Mendy is expected to miss all of those matches.
The defender could be involved in Real Madrid's first match of 2026, which is at home to Real Betis on January 4.
Asensio returns to Real Madrid squad against Athletic
Real Madrid have received a defensive boost ahead of Wednesday's clash with Athletic, though, with Raul Asencio back in the squad after missing out against Girona through illness.
Xabi Alonso's side, who have drawn their last three in Spain's top flight against Rayo Vallecano, Elche and Girona, are second in the La Liga table, four points behind the leaders Barcelona.
Full Real Madrid squad vs. Athletic
Courtois, Lunin, F Gonzalez; Militao, Alexander-Arnold, Asencio, Carreras, F Garcia, Rudiger; Bellingham, Camavinga, Valverde, Tchouameni, Guler, Ceballos; Vinicius, Endrick, Mbappe, Rodrygo, Gonzalo, Brahim, Mastantuono