A soft mattress may have been the reason Marcelo only managed 10 minutes of Brazil's final group game.
Marcelo was replaced by Filipe Luis in the early stages of Brazil's 2-0 victory over Serbia on Wednesday night, citing a back issue as he limped off the pitch in the Spartak Stadium.
Seleção team doctor Rodrigo Lasmar has explained that the left-back was removed as a precaution after muscles in his back seized up, before suggesting that Marcelo's mattress could have been to blame.
"Marcelo has a medical history with this kind of injury," Lasmar said in the mixed zone. "Maybe it was the hotel's mattress, which was a bit more soft than usual. But [the injury] happened because he made a rotation movement and felt uncomfortable after. He could not continue in the game, because his muscles became locked up."
Marcelo's injury is the most recent of several picked up by Brazil players in the past week, as both Danilo and Douglas Costa missed Wednesday's game due to muscle problems suffered in the opening pair of group matches.
Brazil travel to Samara on Monday for their round of 16 clash with Mexico and, as of Wednesday night, it was unclear whether Marcelo would return in time.
Manager Tite said: "I only know that he had a problem with his back. I know it was his back, more than that the doctor can explain. He said he locked up when he was leaving the pitch."
