A soft mattress may be behind the back injury that forced Brazil left-back Marcelo out of his country's 2-0 victory over Serbia, according to the team's medical staff.
Marcelo limped from the pitch early during Wednesday's game in Moscow, being replaced by Atletico Madrid's Filipe Luis, with the score still at 0-0.
Speaking in the Spartak Stadium's mixed zone after the game, Brazil team doctor Rodrigo Lasmar explained that the substitution had been a precaution as the 30-year-old had felt muscles "lock up" and knew he could not continue.
"Marcelo has a medical history with this kind of injury," Lasmar said. "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 has missed games for Real Madrid due to muscle problems in both of the last two seasons, with hamstring trouble in his right leg in February 2018 an issue in the same place of the other leg in January 2017.
Brazil's next game at the tournament is next Monday afternoon's round-of-16 meeting with Mexico in Samara.
Asked after Wednesday's match what happened, Selecao 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."
Marcelo's exit marks the latest in a Brazil side beset by injuries for this World Cup. Manchester City right-back Danilo was ruled out with a thigh problem before Brazil's second group stage game against Costa Rica and has yet to return.
Juventus winger Douglas Costa impressed as a substitute in that 2-0 win against Costa Rica, but missed the meeting with Serbia due to another thigh injury.
First choice right-back Dani Alves was ruled out of the tournament last month after suffering a knee injury while playing for Paris Saint-Germain.