Diego closes in on Juventus move
Media reports, which have also linked Bayern Munich and Inter Milan with the 24-year-old, said Juve had agreed a fee of 25 million euro with the Bundesliga club.
"We think talks are going well. We hope to conclude the deal but it is still not sealed," Cobolli Gigli told reporters.
He also denied Juve were considering selling Italy goalkeeper Gianluigi Buffon.
The Turin club have dropped out of the title race having not won in five Serie A games but coach Claudio Ranieri has been told his job is safe until the end of the season.
Other signings are expected to boost their prospects next term. Winger Pavel Nedved is due to retire at the end of this season.