The Argentine spoke to reporters at his first press conference as a Fiorentina player, addressing his difficult debut campaign in Spain and his motivation for the new chapter in Italy.
"At Madrid we all had a difficult season, it wasn't an easy year," Mastantuono said. "But, as I always say, these things help you improve, learn and realise that there's always room for improvement in football."
He was careful to frame the move as a fresh chance rather than a point to prove against his parent club.
"It's not about revenge, it's an opportunity to try to show the player I am and to help Fiorentina," he said.
Mastantuono revealed that his conversation with Real Madrid over his departure had been "very cordial and friendly," and that he chose the Tuscan club among several options because the project impressed him "a lot."
"I made this decision a week before the announcement, and I'm very happy," he added, saying he felt a responsibility to help take Fiorentina "to the top."
The 19-year-old, who underlined the privilege of having played for both Real Madrid and River Plate, said Fiorentina's supporters had welcomed him in an "incredible" way.
"They've given me a lot of affection, and that reminds me of home, of Argentina. I was playing on the beach and had fun with some kids who were there. For me it's normal to be with friends and family and play on the beach."
Mastantuono singled out Fiorentina legend Gabriel Batistuta as a figure he admires, saying he would love to make history at the club in the same way.
"He played here and did something incredible. But there are also teammates who have been here, that I've shared a dressing room with, like Martínez Quarta and Nico González. I've spoken to them and they've told me wonderful things about the atmosphere here.
"I don't set myself targets, I want to help the team and improve day by day," he concluded.
Mastantuono made 35 appearances for Real Madrid across all competitions last season, 17 of them as a starter, scoring three goals in the process.