De Gea confirmed his return to the top of the game with two penalty saves in victory over AC Milan on the weekend.
Former Viola keeper Viviano told Radio FirenzeViola: "Saving penalties is a coincidence but I'm surprised at those who are surprised. Three years ago he was one of the best goalkeepers in the world and today for Fiorentina he is a luxury, with all due respect for a team I love. It seemed like I was dreaming when I heard that he would compete for a place with (Pietro) Terracciano, but what are we talking about? With all due respect for an excellent goalkeeper like Pietro.
"Then De Gea is not a penalty saver, so he was truly incredible. I really liked the save on (Samu) Chukwueze because there you really risk making a fool of yourself, he was very good at saving the team."
On De Gea's year out since leaving Manchester United, Viviano also said: "It happened to me too to have to sit still, but a goalkeeper is not an outfield player, he just has to find his best form because it's about explosive strength in the moment and little else. After a year you need to find the right sensations and spaces but it's like riding a bicycle, it's not like you forget how to do it. I remember some difficulty with crosses but with training everything goes away and above all your value doesn't change."
On De Gea's kicking, he also said: "It has never been his strong point. Maybe it's what has kept him from becoming a legend because he is one of the best in goal in history, and in England it is perhaps a little more difficult because the charge on the goalkeeper is rarely called. I heard some criticism after the Atalanta match, but come on... It makes me smile.
"Fiorentina is a team that plays man-to-man and De Gea will have to learn to manage the space outside the goal. In my opinion, the club has made a great purchase, as have others: you can discuss the timing but you can't say anything about the market value."
On extending De Gea's contract, Viviano added: "It depends on him, because now the spotlight is on him again. Because if Arsenal arrives, it will be difficult to convince him. Who knows, however, that he might become a capital gain for Fiorentina."