Marcelo picked up a knock in Real Madrid's comprehensive loss at Sevilla, with head coach Julen Lopetegui awaiting further details after the left-back suffered an apparent muscle tear.
Madrid not only lost 3-0 to Sevilla in LaLiga but the Champions League holders saw Marcelo forced off due to injury on Wednesday.
Marcelo hobbled off the pitch with 18 minutes remaining, leaving Madrid down to 10 men during the closing stages after all three substitutions were made.
With Saturday's derby against Atletico Madrid looming, there are question marks over Marcelo's fitness.
"I picked up a knock, but the defeat hurts more," Brazilian star Marcelo told reporters.
FP: 3-0 (André Silva 17', 21', Ben Yedder 39') September 26, 2018
Asked about Marcelo during his post-match news conference, Lopetegui said: "He took quite a blow, but we'll see what we can find out tomorrow."
It was a night to forget in Seville, where Madrid tasted defeat in LaLiga for the first time under Lopetegui following Andre Silva's brace.
Luka Modric had a goal disallowed by VAR early in the second half, ending Madrid's hopes of a comeback away from home.
Madrid missed the opportunity to capitalise on Barcelona's shock loss at Leganes, with the Spanish giants level on points atop the standings.
"You have to admit Sevilla were better in the first half," Marcelo said. "We got more of the ball in the second half but they were dangerous on the counter. They are a tough side and we lacked a few things against a rival as strong as Sevilla are.
"In the second half, maybe if Luka's goal had stood, the game would have been different. If you lose, your drop three points and that can make the difference later in the season.
"We tried our best and let’s see if we can avoid such errors in the future. We need to change things now, defeat hurts, but we have to pick ourselves up or the next game."
