The 18-year-old signed for the Nerazzurri from Dinamo Zagreb on deadline day for a fee believed to be in the region of 11 million euros.
And the former Milan striker believes Kovacic will soon prove to be a shrewd acquisition.
“I saw him in the first training session, and you can tell he's got great ability,” the 30-year-old told the club's official website.
“Inter have spent big and spent well on him – he's going to be here for the next 10 years.
“He'll become a great player, I'm 100 percent sure of that.”
Meanwhile, Cassano also addressed the comparisons being made between him and Milan's Mario Balotelli and believes the latter's propensity to attract the media spotlight is among the distinctions.
He added: “There's one difference between Mario and I – he probably likes being in the paper, whereas I don't.
“But I've done worse things in my life. I left training, I drove my car onto the pitch – you name it, I did it.
“He's a good lad though."