The veteran Croatia striker has been linked with Manchester United in the local Italian press in the past week.
But De Vito, an expert of the Croatian transfer market, is convinced Allegri will not allow Juve to part with him.
He told Tribalfootball.com: "Mandzukic has become a complete player, thanks to Allegri who used him in different positions. Mandzukic is now a unique player.
"Allegri will insist he stays, I don't think Juventus will sell him.
"AC Milan could try, but it is difficult."
De Vito is also close to Juve midfielder Marko Pjaca, who spent the second-half of this past season on-loan in Germany with Schalke. He is convinced the Croatia international is destined for big things,
The Italian agent added: "Pjaca is a pure talent, he only needed continuity and at Juventus he couldn't find it, having Douglas Costa, (Paulo) Dybala, Mandzukic, (Federico) Bernardeschi ahead of him.
"The decision to go to Germany was the best one, he grew up, now he has more personality and he is technically stronger. He will be a big thing, I don't know when, but I am sure he will.
"I know him and he has a wonderful repertoire."