Rubinho, now an agent, believes the former Manchester United keeper will improve the Viola at the back.
He told Radio FirenzeViola: "De Gea is still suffering from having been out for more than a year, but he has already helped Fiorentina a lot in these first games.
"I think he can only grow further and show everyone the qualities he has. He is a top goalkeeper and seeing him defend Fiorentina's goal reminds me of when there were the likes of (Francesco) Toldo and (Sebastien Frey.
"Goalkeepers need to feel confident on and off the pitch. (Pietro) Terracciano was a goalkeeper who, even in a match in which he played well but at the end made a mistake, the fans criticised him.
"De Gea, on the other hand, even if he doesn't have a top game, is still De Gea. The Spaniard has the trust of the environment because he gives a lot of security. Terracciano didn't have that much confidence."