Although many expected him to be registered directly with the senior squad, Real Madrid chose to list Mastantuono with Real Madrid Castilla. This means he will wear the No. 30 shirt, since RFEF rules require players registered with the reserve team to use numbers above 25. However, this does not prevent him from playing with the first team. Just as happened with Vinícius Jr. and Rodrygo in their early years, Mastantuono will be able to alternate between Castilla and the senior side throughout the season.