"Following the tests carried out today on Marcelo by Real Madrid's Medical Services, he has been diagnosed with an injury to his left adductor muscle," the club said in a statement on Friday. "His recovery will continue to be assessed."
It's the fourth muscular injury the Brazilian has suffered this season.
Fellow left back Ferland Mendy -- signed from Lyon last summer -- replaced him on Friday, having featured frequently this season. Highly-rated academy product Miguel Gutierrez, 18, was also in the squad.
"Marcelo has a little injury, let's hope it's little," coach Zinedine Zidane said in a news conference on Thursday. "That's why he didn't train this morning. We'll see in the next few days exactly what it is."
Marcelo is expected to return in time for the resumption of Madrid's Champions League campaign.
UEFA confirmed on Friday that Madrid's round-of-16 second leg with Manchester City will be played in Manchester on August 7.
The first leg ended in a 2-1 defeat for Madrid at the Bernabeu in February before the competition was put on hold.
A quarterfinal against either Lyon or Juventus would follow in Lisbon on August 15, with a possible semifinal on August 19 and the final on August 23.