Lamb told Sky Sports News: "Who knows whether Stewart will be here in August. If he is he will be playing for us in the Championship.
"But if the right offer comes along he will be allowed to leave."
Lamb is confident the 24-year-old will make a full recovery from his injury.
He added: "It has been suggested he could be out for as long as six months but he is quite a good healer and we hope he will be back before then.
"There is no reason why Stewart won't make a full recovery.
"He has been told that by the surgeon and he believes it. If someone wants to take a chance on him now, it's up to them. It's out of our hands."