Real Madrid have confirmed that Trent Alexander-Arnold suffered a quadriceps injury on his left leg in their win over Athletic Club on Wednesday.
He went off in the 55th minute at San Mamés and was replaced by Raúl Asencio.
"Following tests carried out today on our player Trent Alexander-Arnold by the Real Madrid Medical Services, he has been diagnosed with a muscle injury in the rectus femoris of his left quadriceps. His recovery will be monitored," the club said in a statement on Thursday.