The star striker had to be withdrawn late on in the tense last eight clash in Fortaleza on Friday.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.
