Marcelo limped off the field during the opening stages of their 2-0 victory over Serbia which qualified the Selecao for the last-16 of the global showpiece.
Visibility distressed upon being replaced by Felipe Luis, the blow caused real concern for head coach Tite and the rest of the Brazil squad due to his attacking presence and experience he brings to the national team.
However, the five-time UEFA Champions League winner's injury appeared a bizarre one - as limited contact was made before his exit.
And the South American giants team doctor Rodrigo Lasmar claims the unspecified injury was caused by a hotel bed.
"(Marcelo's injury may have been caused by) the mattress at the hotel where we were staying in Moscow," he said as quoted by The Sun.
"It's still early to talk about the evolution of the problem. He has responded very well to the physiotherapy treatment that we carried out in the stadium, but we have to wait 24 hours to make a better diagnosis."