Man United, Real Madrid, and, of course, Chelsea, are all interested in the 17-year-old, who is widely regarded as one of the best youngsters in World football.
The young playmaker reportedly has a €45m (£38.4m) release clause in his contract with the Buenos Aires club.
Mastantuono has impressed at River Plate so far this season, scoring three goals and providing two assists in his 12 games across all competitions.
Fernandez, who also came through the River Plate academy, is certainly impressed with the youngster, suggesting he has the potential to go to the very top.
Speaking to ESPN, he said: "He's showing that he has a European future. You can see it.
"He's an incredible player. He's doing very well and has shown great talent. In my opinion, he's a star. I wish him the best and hope he can achieve all his goals. Hopefully he can also help us in the national team."