The 16 year-old has burst onto the scene in Argentina this season.
According to local journalist Hernn Castillo, "Real Madrid is desperate to take him... Mastantuono turns 18 and leaves River, it is inevitable."
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Mastantuono made his senior River debut this year and his deal carries a 30m buyout clause.
Mastantuono (Azul, Buenos Aires; 8-14-2007) is a left-footed midfielder who became the youngest scorer (16 years, 5 months and 24 days) in the history of River Plate, surpassing the record of Javier 'El Rabbit' Saviola (16 years, 10 months and 7 days).