Former Newcastle defender Boumsong said: "I'd say Karim is worth at least £20m at this moment, but any club wishing to buy him this summer will have to pay more.
"He is a fantastic player and if he continues to improve at the rate we have seen, in another year he will be worth £30m. In two or three years, it will be a lot more. Our president has said that he wants Karim to stay for another two years at least. He wants to win the Champions League before selling him and is maybe looking to keep him until after the next World Cup.
"If Karim leaves before then it will be because of money. We cannot compete with Man U at Lyon - they have much more money. But our president is a clever man. He sold Michael Essien to Chelsea for £24m and Karim is a much better player.
"It is good from our perspective that the president wants to keep Karim and the player has also said he is happy at Lyon.
"But if United were interested in Karim before, they will be desperate to sign him after his performance against them in the Champions League."