The Black Cats are preparing themselves for a mass exodus following back-to-back relegations which have seen the club fall from the Premier League to League One in just two seasons.
N'Dong, who spent the second half of last season on loan at Watford without making a single appearance for the club, is expected to be the latest to leave the Stadium of Light following talks with Serie A club Torino.
The 23-year-old is understood to prefer a move to the Premier League, though, which is delaying the transfer as he waits to see if any interest arrives from England.
Cardiff City are thought to be weighing up an approach for the Gabon international, although Torino are eager to push through the £6.6m deal as soon as possible.
N'Dong joined Sunderland in 2016, going on to make 54 appearances for the club across all competitions.