Real Madrid captain Dani Carvajal has suffered a calf injury, the club said on Sunday, with Spanish media reporting that the defender will be out for at least four weeks.
He has played in seven of Real's eight games in all competitions this season but had to come off after an hour in Saturday's 5-2 derby loss at Atletico Madrid in LaLiga.
The 33-year-old had returned to action in July for the Club World Cup semifinals after nine months out with an anterior cruciate ligament injury.
"After the tests carried out today on our player Carvajal by the Real Madrid Medical Services, he has been diagnosed with an injury to the soleus muscle of his right leg," Real said in a statement.
The club did say how long he would be out but newspaper Marca said Carvajal will be sidelined for around a month, meaning he is likely to miss Barcelona's visit on October 26.
Fellow right back Trent Alexander-Arnold and centre back Antonio Ruediger are also injured, leaving manager Xabi Alonso short of options in defence.
Real next travel to Kazakhstan to take on Kairat Almaty in the Champions League on Tuesday.