Real Madrid fear "really bad news" on the Marcelo injury front after the Brazilian defender picked up a knock 30 minutes into Sunday's 5-3 win over 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 to be in severe discomfort on the pitch as it became clear he would not be able to play on.
Now the club face a nervous wait, with director of institutional relations Emilio Butragueno fearing "really bad news."
"We need to wait and see what the doctors say," he told AS after the match. "He's had to leave the pitch and it's really, really bad news. In 48 hours we'll know the exact extent of the injury."
The injury comes at a tough time for Madrid, who face four La Liga matches between now and March 6, when the club will travel to Paris to take on PSG in the second leg of their round of 16 Champions League clash.
Manager Zinedine Zidane is hopeful that the injury won't be anything major moving forward.
"Hopefully it's something small," he said. "Tomorrow we'll see. (Marcelo) said that the sensation is that it is small, but we will see in a few days."
Madrid hold a 3-1 lead over the French outfit after the first leg and sit fourth in La Liga, 17 points back of leaders Barcelona.
Marcelo has scored three goals over 28 matches in all competitions this season.