Filipe Luis is set for a spell on the sidelines after suffering a thigh injury during training on Wednesday, Atletico Madrid have announced.
The left-back has been a mainstay in Atletico's defence under Diego Simeone, playing 18 of their 20 matches in all competitions this season.
He sat out Tuesday's 1-0 defeat to Bayern Munich in the Champions League with Atleti's progress to the last 16 already secured and a day later he suffered a grade two tear in his left thigh.
Tests conducted on Thursday confirmed the tear, but Atleti did not confirm how long he will be out for.
"Filipe Luis suffered a grade II injury to the semimembranosus muscle in his rear left thigh," a club statement read.
"Our player, who retired from training on Wednesday with discomfort, had medical tests on Thursday at the Majadahonda Clinic.
"The Brazil international will begin physiotherapy treatment with alternative training. Evolution is pending."
Atletico look set to be without the 31-year-old for their next LaLiga clash with Villarreal due to the injury.