Brazil's team doctor Rodrigo Lasmar has claimed that a hotel mattress could have caused the injury that forced Marcelo to miss the majority of Wednesday's 2018 World Cup clash with Serbia.
The Selecao ensured that they would top Group E with a 2-0 win over Serbia in their final group fixture.
Real Madrid's Marcelo came off after 10 minutes of the match, however, with the full-back suffering back spasms in the build-up.
Dr Lasmar admitted that it was "still too early to make an accurate diagnosis", but has suggested that the defender will be fit for Brazil's last-16 clash with Mexico on Monday.
"It is still too early to make an accurate diagnosis," Dr Lasmar told reporters. "The injury may be related to the hotel mattress. Although our expectation is good, but we have to be calm and wait for the next few days.
"Marcelo is medicated and with medication the tendency is that the response is good and that he can play. The important thing is that there is no injury. They were muscle spasms. Once he feels good to train, he will return and immediately be ready to play. There is no recovery period."
Brazil collected seven points from their three group matches at this summer's competition.
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