United won 4-0 at Swansea today.
Hartson told BBC Sport: "Swansea need a lift.
"They need players to come in the door; Paul Clement, I am sure, will have his eyes on several players.
"But you cannot keep losing your best players every year. We saw how close Swansea came to going down last year. They lost Ashley Williams last summer, Wilfried Bony the season before.
"Now they've lost Gylfi Sigurdsson, you cannot keep recovering year in year out when you sell your best players.
"There's money here for sure."