The experienced left-back sustained the problem during the 1-0 win over Athletic Bilbao on Sunday, a game in which he won the penalty that resulted in the decisive goal.
LaLiga leader Madrid announced in the lead up to Saturday's (AEST) clash with Deportivo Alaves that the Brazilian's injury had been diagnosed as a problem in his left adductor muscle.
While the club refused to put a timeframe on his recovery, media reports suggest it does not expect him to be available for the three remaining league games after the visit of Alaves.
Zinedine Zidane will be without Marcelo for trips to Granada and Leganes either side of a home match against in-form Villarreal.
He is not thought to be a major doubt for the UEFA Champions League Round of 16 second leg against Manchester City on 8 August (AEST), however.