Brazil star Neymar left national team training on Wednesday night after speaking with a team doctor, according to multiple reports.
The Barcelona forward grabbed his right knee after shooting 20 minutes into training before limping off the training pitch in New Jersey, reports said.
According to TV3, Brazil manager Dunga decided not to risk the player for the rest of training and told him to rest.
Neymar just returned from two weeks on the sideline after contracting mumps. He made his season debut for Barcelona in Saturday's win over Malaga.
Brazil are preparing for two friendlies, on Satuday against Costa Rica at Red Bull Arena and against the United States next Tuesday in Foxborough, Massachusetts.
Neymar is eligible to play in the friendlies but will be suspended for Brazil's first two World Cup qualifiers in October against Chile and Venezuela for his part in a scuffle at the Copa America.