Boyhood Swans fan Hartson is resigned to seeing 21-year-old James leave the Liberty Stadium, with a clutch of clubs, including Manchester United and City, Newcastle and Bournemouth leading the chase.
Welsh legend Hartson, 44, said: “In recent years, Swansea have sold a complete team.
“If they are offered in excess of £20million for Daniel, then it's difficult to turn down.
“It would also be hard to deny the boy the opportunity of playing for a Manchester United. He was very close to going to Leeds on the final day of the January transfer window for £10m.
“But I believe Swansea pulled the plug on the deal at the last minute because they thought, at that price, it was a steal. The transfer was almost over the line. I believe his parents were at Elland Road with the lad, but Swansea then told Leeds the deal was off.
“It wasn't very nice for Daniel because he had probably pinned his hopes on joining Leeds, who then looked like they were on their way to the Premier League.
“The way things have developed could be better for both Swansea and the player.
“From the club's point of view, the transfer fee will at least be doubled – while Daniel could join one of the Premier League's top clubs."