The Senegalese centre-back has been linked with a move to Manchester City, which will finish as runner-up to Liverpool in the Premier League.
City successfully appealed against a ban from UEFA competitions and may be an attractive next club for Koulibaly, who has excelled in Naples.
However, Napoli is not rushing to sell, even though any deal could help to fund its own rebuilding plans before next season gets under way.
Giuntoli said: "At the moment there are no real offers. We are in no hurry to decide now. We will evaluate with the player what to do."
Koulibaly, 29, may leave while Napoli strengthens its attacking options with the signing of Victor Osimhen from Lille, with the Nigerian striker a confirmed target.
Giuntoli said: "We are interested, but the negotiation is not as far forward as everyone thinks."