Gianluigi Donnarumma only turned 18 on Saturday but the Italy international has already made 55 appearances for AC Milan.He made his debut for the Rossoneri at the age of 16 years and 242 days - making him the second youngest goalkeeper to start a match in the history of Italian football - and is expected to become one of the world’s most in-demand players if he continues his current career trajectory.The Mirror have reported that Manchester United are keeping close tabs on the teenager, whose market value is already estimated to be around the £40 million mark, in case David de Gea leaves Old Trafford for Real Madrid. However, the Red Devils are likely to face stiff competition from their local rivals Manchester City as well as a host of other top European clubs.Back in September, the great Gianluigi Buffon paid Donnarumma the highest possible compliment when he named him as his successor.“Donnarumma is a chosen one,” the Italian football icon was quoted as saying by the Daily Mail. “There's a new project, and we're ready to make our quality available to the coach.“When you feel you still have something to give then you're ready for a new project.”

Buffon sent a message to Donnarumma this weekend

And Buffon, classy as ever, paid a special tribute to the youngster on his 18th birthday this weekend.

"Hello Gigio, It's Gigi. I wanted to wish you a very happy birthday, finally entering the world of greats as the great player you are," Buffon said in a video message released via Sky Sport Italy, per FourFourTwo.

"For this, enjoy your age in the best possible way, the age of maturity.

"You will understand that the world of greats is a difficult one but you have all the qualities, moral and human ones, to create an important space [for yourself], to be happy and define an era."

Milan's tribute left Donnarumma in tears

However, it was another special birthday tribute that left Donnarumma in tears.

AC Milan put together a short video of various people at the club wishing the keeper a happy birthday and it was all a bit too much for the young lad.

Donnarumma might be 6ft 5in tall but this footage proves he’s a gentle giant, really.

Lovely stuff.

Video: Donnarumma's best saves this season

Milan also released a video of some of Donnarumma’s best saves from this season so far…

What a talent.