The highly-rated stopper became the youngest goalkeeper ever to debut in Serie A at the age of 16, and has since become Milan's number one despite only turning 17 earlier this year.
The teenager, who stands at 6'5", also became Italy's youngest-ever goalkeeper when he made his international debut in September's friendly with France.
"I'm really impressed, I don't know about surprised, but I'm very, very impressed with him," Hart told Goal.com. "He just seems to perform every single week. [Just] 17 years old, [and he's] been doing it since he was 16... that's young for a goalkeeper to play at this level.
"He's not just interesting to watch because he's young, he's interesting to watch because he's actually very good and that's a big compliment to him and if he continues he's going to be... he's already a special talent, the sky's the limit for him."
Real Madrid and Chelsea have both been credited with an interest in the teenager, but it has recently been reported that the goalkeeper wants to sign a new Milan deal on his 18th birthday.