Roma signed the starlet in April 2014, in a deal that could net his former employers up to $2.5 million, depending on appearances and performances.
De Silva said goodbye to his Glory teammates at a farewell presentation on Monday. The 18-year-old said he was excited about the prospect of playing football for one of Europe's biggest clubs.
"It's starting to really hit home, but you know, I'm really excited and looking forward to what the future has in store," De Silva said.
"It'll be fantastic for me be able to learn.
"Obviously I've already been learning off some of the great players in the Socceroos but to learn off some of the world's best players like Totti, De Rossi, Gervinho - all those players, it'll be fantastic for me and it'll help my football develop as well."
He made 32 appearances in total for the Glory, scoring three goals.