Real Madrid have confirmed Marcelo suffered a hamstring injury in the crushing El Clasico defeat to Barcelona on Sunday.
Julen Lopetegui was sacked just over 24 hours after the humiliating 5-1 hammering and the fallout has continued with the loss of a second key defender.
Raphael Varane sustained an adductor issue at Camp Nou and Brazil international Marcelo will join him on the sidelines for an indefinite period.
The 30-year-old scored Madrid's only goal before being withdrawn in the 82nd minute, leaving him in doubt for Saturday's LaLiga home meeting with Real Valladolid.
"Following tests carried out on Marcelo by the Real Madrid Medical Services, he has been diagnosed with a muscular injury to his right hamstring," read a club statement.
"His recovery will continue to be assessed."
Madrid had already confirmed Marcelo would play no part in interim boss Santiago Solari's first game in charge against Melilla in the Copa del Rey on Wednesday.
The left-back hurt his calf in late September and missed three matches leading into the last international break.