Neymar was taken to hospital after being carried off on a stretcher during Brazil's 2-1 FIFA World Cup quarter-final win over Colombia.
The star striker had to be withdrawn late on in the tense last-eight clash in Fortaleza on Friday.
And early reports in Brazil suggest that the injury, believed to be in his back, could bring a premature end to his tournament.
The news puts a further dampener on the victory for Luiz Felipe Scolari's men, with Thiago Silva already out of the semi-final versus Germany due to suspension after being booked against Colombia.
However, Neymar's team-mates were attempting to put a positive spin on a potentially devastating blow.
"Neymar is a warrior and I'm sure he'll be fine," said misfiring striker Fred.
Captain Silva opened the scoring on the night, bundling home after seven minutes before David Luiz's stunning second-half free-kick made it 2-0.
Brazil held on superbly after James Rodriguez reduced the arrears from the spot, although thoughts had soon moved away from their triumph and to Neymar.comments