The 18-year-old only joined Real Madrid from River Plate for a reported €63.2 million, including add-ons, last summer.
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Mastantuono struggled for regular first-team football, however, starting 11 of his 21 LaLiga games and scoring just one goal.
The arrival on Yan Diomande would likely make those chances even more scarce, so Real Madrid decided to ship him out on loan to Fiorentina.
The agreement between the two clubs does not include a purchase option, according to Marca, so the player will return to Real Madrid next season.