Real Madrid defender Marcelo suffered an injury to his left adductor muscle during Saturday's 4-0 defeat against rivals Barcelona at the Santiago Bernabeu.
The Brazilian full-back was one of few Madrid players to impress in the Clasico reversal but was withdrawn prior to the hour in the capital.
And, after a difficult period for the club, who have since seen coach Rafael Benitez and president Florentino Perez come in for criticism, Marcelo will now miss Wednesday's Champions League clash with Shakhtar Donetsk.
"Following the tests carried out on Marcelo today in the Sanitas la Moraleja University Hospital, the player has been diagnosed with an injury in his left adductor muscle," read a Madrid statement.
"His recovery will continue to be assessed."
Benitez's men are back in La Liga action on Sunday with a trip to Eibar.
