Brazil midfielder Fred, who is close to joining Manchester United, injured an ankle in training a week before the start of the World Cup.
United have agreed a £47m deal for the 25-year-old Shakhtar Donetsk player, with his move to Old Trafford expected to be completed soon.
Brazil team doctor Rodrigo Lasmar described the injury, which happened in London, as a “trauma in his ankle”.
Brazil begin their World Cup campaign against Switzerland on 17 June.
The five-time winners play further Group E matches against Costa Rica and Serbia in Russia.
Lasmar added: “At the moment it is too early to say anything about him.
“On Friday, we watch to see what happens and then decide if we need to do any exams or not.”