The former Manchester United midfielder has been linked with a return to the Premier League this season.
However, Manna insists there's no plans to part with the midfielder.
He told Sportmediaset: "Scott is an important player for us, he's happy to be in Naples and it shows when he plays and experiences the city.
"He has two years left on his contract, we have a clear and frank relationship, we're talking, but it's not a current topic.
"We know Scott's importance, we haven't received any offers, partly because the player has never expressed the idea of moving away."
Manna also said: "Our goal has always been to qualify for the Champions League. This is crucial not just for Napoli, but for all the competing teams.
"We're happy with what we're doing; we're extremely positive. I see the team improving, and we're bringing back key players who should have been available from the start of the season, but who unfortunately haven't been available."