Star striker Erling Braut Haaland is out until January with a leg injury, while midfielders Thomas Delaney, Raphael Guerreiro, Mahmoud Dahoud and Mateu Morey are all missing with various ailments.
Reinier, 18, arrived in Spain from Flamengo, with whom he won the Copa Libertadores, the South American equivalent of the Champions League, and the Brazilian league last season.
LaLiga: Emotional and tearful 18-year-old Reinier Jesus presented as Real Madrid player at Santiago Bernabeu
Reinier is the third Brazilian teen signed by Madrid in recent seasons, following Vinícius Júnior and Rodrygo. Vinícius also played for Flamengo
Tokyo Olympics 2020: South American football teams to play pre-Olympic qualifying tournament without best players
For the last three Olympics, the South American Youth Championship was used as the Olympic qualifying event instead