The reigning Serie A champions have seen transfer talk start to build ahead of the winter window.
Argentine forward Dybala is among those to have been linked with a move elsewhere, with the 24-year-old conceding that he “can’t say this will not be my last year in Turin”.
Premier League title hopefuls United are among those said to be keen on a man who continues to draw comparisons with Barcelona superstar Lionel Messi, but Casiraghi cannot see a sale being sanctioned any time soon.
The former Juve striker told bwin: "We’re not even at the half of the year, it’s too early. He had a good start, now it’s going worse, but he has to be as decisive as at the beginning of the season.
"Juventus are a great team, I don’t think they’ll lose such an important player."
While Dybala, who has 13 goals in 18 appearances this season, is being linked with an exit, Arsenal full-back Bellerin is being mooted as a possible reinforcement for Juve.
Again, though, Casiraghi doubts that a deal could be done for a 22-year-old talent who is tied to a long-term contract in England.
He added: "He's a good player, I know him well, but it’s difficult to see an English team losing such an important player.
"Juventus will need so much money, and it’s difficult to fight with these teams. They have much more money than our teams."
An approach for Liverpool midfielder Emre Can is more feasible for Juve, with the Germany international seeing his current contract run down at Anfield.
He has been heavily linked with a switch to Italy, but Casiraghi has questioned whether Massimiliano Allegri needs another player in that area of the field."We still have to wait for June. Juventus is a complete team, the midfield is full of quality.
"[Miralem] Pjanic is a great player, there are [Blaise] Matuidi, [Sami] Khedira and [Claudio] Marchisio, finally recovered. There are so many players, it’s not so urgent. In June the situation is different."