Real Madrid will not allow Vinicius Jr. to join Brazil's under-20 side for the South American Youth Championship in Chile in January and February, according to U20s coach Carlos Amadeu.
The tournament does not fit into a FIFA international window and thus clubs are not forced to free players for their national teams.
"Real Madrid has already made it very clear to us that the players that are part of the squad will not be released for the South American tournament but they will be for the [FIFA U-20] World Cup," Amadeu told Globo Esporte. "That's the stance."
Vinicius, 18, joined up with Brazil's U20 team last week and started but did not score in their two friendly draws against Chile.
He is hopeful Madrid will reconsider their decision and allow him to return to compete in the tournament.
"We all know it's very difficult, right, because it is not a FIFA date, and the club is not obliged to release [players]," Vinicius said. "But we are trying not only for myself but for all the players to play the South American [tournament] and play for the national team."
Four teams will qualify for the FIFA U-20 World Cup, which starts in May and will be held in Poland, from the South American Youth Championship.
Vinicius joined Madrid this summer after the club agreed a €45 million transfer from Flamengo in May 2017.
He made four first-team appearances in preseason but has made just two league appearances for Madrid, both as a substitute, and has spent time playing for Real Madrid Castilla, the club's reserve side.