Preston chairman Derek Shaw told the Sunday Mail: "Celtic's offer was way below our valuation. There's no way we'd accept anything near the figure put to us. We are not being greedy.
"Sean is an established international and players of his calibre come at a price.
"I know a Championship club that has put a £4million price tag on a defender who only has six months left on his contract.
"But we understand Sean wants to play at a higher level and has his international career to think of. So the ball is in Celtic's court."