The 17 year-old midfielder has moved to Real from River Plate for €63m.
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Mundo Deportivo says Mastantuono will receive a salary of €3.5m, which is close to that of another potential future star, Endrick, who is paid €4.1m-a-year.
Mastantuono, now 17, played 22 matches for River Plate in all competitions this season, in which he has scored seven goals and provided four assists.
The midfielder is viewed as a first team prospect for coach Xabi Alonso this new campaign.