The youngster, who is only 16, is already on the radar of clubs such as Manchester United.
Per O Jogo, United have been scouting the talented attacking midfielder in recent weeks.
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United do face competition if they want to sign Mastantuono, as he is wanted by a host of top teams.
The likes of Real Madrid and Barcelona are also circling, while other teams are also in the mix.
Mastantuono has played 10 times for River's senior team so far, netting one goal.