Championship leaders Southampton have recently had two offers for the 23-year-old turned down by the Bhoys, with a third offer expected later this week.
However, Lennon has told the Saints that they will have to up their bid significantly if they want to add the former Scunthorpe frontman to their squad.
"Gary is happy here. He has good feet and is a good finisher," said Lennon. "But if I had to put a price on Gary's head then it would be £35m.
"Even if we did receive a knock-out price for Gary, would we be able to replace him. Trust me, goalscorers of his calibre are not easy to find.
"We're top of the league and that brings its own pressure. But I hope we can keep all our good players."
Hooper joined Celtic in 2010.