Modena have sold the youngster to Inter, though Massolin remains on-loan with the Serie B club for the rest of this season.
Catellani told Sky Italia: "He's a special player, with great physicality and great technical ability.
"He's one of those players you don't often see on the pitch: Inter's investment was far-sighted. There's also great satisfaction for the Serie B movement; it's an important sign."
He also said of the midfielder: "He's a player who needs to complete his development. Tactically, he's performing well as a midfielder, but he's also played as an attacking midfielder behind the strikers.
"He's also reminiscent of Josip Ilicic. He's highly intelligent, both on and off the pitch: this is the most important guarantee for the future.
"It will be a great challenge, and we hope he can help us achieve our goals in the coming months."