The 24-year-old began his career in Italy at Inter, although he was loaned out to Lecce, Padova and Grosseto.
The defender moved to Leverkusen at the start of the 2013-14 campaign, and made 23 league appearances as they finished fourth in the Bundesliga.
And it has emerged that Donati, capped by Italy at Under-21 level, has become the subject of interest from Fiorentina.
However, Voller revealed that the German club would be reluctant to sell.
"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."
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