Vinicius was forced off with a thigh injury during Brazil's 2-1 loss to Colombia in a World Cup qualifier on Thursday, and he returned to his club for further checks.
"He has been diagnosed with a tear in the femoral biceps with involvement of the distal tendon in his left leg. Evolution pending," Real Madrid said in a statement.
"We will come back stronger," he wrote on social media platform X, with media reports saying the Brazilian is likely to be out for around 10 weeks. Vinicius has six goals and four assists for Real in all competitions this season.
Real Madrid have also lost midfielder Eduardo Camavinga to injury on international duty when he hurt his right knee while training with France.
The forward had played in every league game for Real this season and he joins Thibaut Courtois, Eder Militao and Arda Guler on the list of Real players who have suffered knee injuries.
Real are second in LaLiga, two points behind Girona. They travel to Cadiz next Sunday when the league resumes after the international break.
