The Saints have been tracking the French midfielder since February and will step up their interest this summer as manager Ralph Hasenhuttl wants him to replace want-away captain Pierre-Emile Hojbjerg.
Celtic are said to be prepared to let Ntcham leave, with the 24-year-old declaring last summer that he wants to play in a more competitive league in order to boost his chances of breaking into the France national team.
A move to Southampton would offer him that, and Celtic could get Forster in exchange.
The 32-year-old spent the just concluded season on loan at Parkhead, playing a huge role in their Scottish Premiership victory, and boss Neil Lennon wants to keep him for next season’s 10-in-a-row bid.
He has two more years left on his Southampton deal, but the club are willing to get him off the books, and both clubs could be willing to exchange players in a deal that might please all the parties involved in the summer transfer window.