Real Madrid face a nervous wait to find out the extent of Marcelo's injury after the full-back lasted just 30 minutes of Sunday's clash against Betis.
The Brazil international started again for Madrid after scoring midweek against Paris Saint-Germain.
But just before the half-hour mark he attempted to play his way out of a tight spot deep in his own half, and went down holding his right hamstring without being touched by a Betis player.
Marcelo originally tried to continue, but looked in severe discomfort on the pitch as it became clear he would not be able to play on.
Theo Hernandez was chosen to replace the left-back, who must now wait to find out what damage has been incurred to the muscle at a pivotal point in Madrid's season.
Madrid began Sunday 20 points behind Liga leaders Barcelona, albeit with two games in hand over their arch-rivals.
As well as Marcelo the Merengue have been shorn of the services of Toni Kroos this week through a knee sprain, with centre-back Jesus Vallejo the only other current casualty in the squad.