Ferrero sold the Czech Republic striker to Roma last summer.
He told TMW: "At Roma I gave a real champion, a natural talent, a humanly crazy guy, only time will give me reason: he has not trained and they threw him into the fray, there is so much pressure in Rome, let's give him time and he will have fun.
"He will be worth €100 million."
On AC Milan striker Patrick Cutrone, Ferrero also revealed: "He's a great guy, he will give a lot of satisfaction, I tried to ask Milan but they did not give in. I looked for him before he scored all these goals."