Brazil winger Raphinha suffered a hamstring injury in the 3-0 win over Haiti and it is unclear how long he will be out.
The Seleção face Scotland in their final group match on Wednesday in Miami, with knock-out progression not yet secured.
Tests confirmed that Raphinha had suffered "a muscle injury in the back of his right thigh," the Brazilian association said.
The 29-year-old will undergo an "intensive treatment programme." The aim is for him to be fit again as soon as possible.
Raphinha was substituted shortly before half-time on Friday.
Veteran star Neymar, who has recently been struggling with a calf injury, will likely be available again for the group stage finale against Scotland, coach Carlo Ancelotti said.
