The 24-year-old, who joined from Inter Milan last summer, made just 13 Bundesliga appearances in his debut season in Germany and has subsequently been linked with a return to Italy.
Donati is a former Italian Under-21 international, although he never featured in a league game for Inter as the majority of his four years there were spent out on loan at Lecce, Padova and Grosseto respectively.
Voller says that Fiorentina have been in touch regarding Donati, but insists that he is a part of new manager Roger Schmidt's plans for next season.
"It’s true, (Fiorentina director Daniele) Prade has phone me and asked about the availability of Donati," Voller told BILD.
"We need Donati, we have a season full of matches ahead of us. This is also the point of view of Roger Schmidt. The manager regards Donati highly."