The Spanish giants are unsure how long the 32-year-old will be unavailable for as their defensive problems worsen towards the end of the season.
Marcelo played all 90 minutes of Los Blancos' 1-0 victory over Athletic Club on Sunday, but was sent for a scan at the club's medical center on Friday which confirmed the injury.
"Following the tests carried out today on Marcelo by the Real Madrid Medical Services, he has been diagnosed with an injury to his left adductor muscle," a statement on the club's website read.
"His recovery will continue to be assessed."
The Brazil international could be out for the remainder of Madrid's La Liga campaign, as they have just four games left to go.
Marcelo medical report. #RealMadrid— Real Madrid C.F. 🇬🇧🇺🇸 (@realmadriden) July 10, 2020
Marcelo has been limited to just 15 appearances in the Spanish top flight in 2019-20 having suffered neck and calf injuries throughout the campaign, while coach Zinedine Zidane has opted to pick Ferland Mendy ahead of him on occasion. The ex-Fluminense player could be fit on time to face Manchester City in the second leg of their Champions League last-16 tie, which will be played next month.