The central defender has been a target for a host of top European clubs in recent seasons, with City and rivals Manchester United among them.
City have been in the market for a commanding central defender since losing Vincent Kompany at the end of the 2018-19 season and their hopes of landing Koulibaly have been boosted.
"Koulibaly and Milik are on their way out but we need to see what offers arrive" De Laurentiis told reporters. "If they go, we know how to replace them. If not, we'll discuss it again next year."
Koulibaly was signed from Genk in 2014 and, after a difficult start, established himself as one of the game's leading central defenders under the guidance of former manager Maurizio Sarri.
Milik was signed as a replacement for Gonzalo Higuain in 2017 and has showed glimpses of his goal scoring talent, but his time in Naples has been blighted by a series of serious knee injuries and he is a target for Serie A sides Roma and Juventus.
Napoli have shown they have the ability to adequately replace their most important players when they move on.
In 2018, they lost Jorginho -- who was central to their passing style of play under Sarri -- to Chelsea and brought in Fabian Ruiz. The Spain international is now a target for several top clubs across Europe.
Meanwhile, the City Football Group has bought a tenth outfit after French club ESTAC Troyes confirmed they were now under the organisation's ownership.
As well as Man City and Troyes, they own New York City FC, Girona, Melbourne City, Montevideo City Torque, Sichuan Jiuniu,Yokohama, Mumbai City and Lommel