Southampton have had a second offer for Celtic striker Gary Hooper turned down by the Scottish Premier League club.
"Southampton's valuation of the player and our valuation are some way off each other," Lennon told the Daily Record.
"I've no idea if they'll come back in for Gary a third time, but they'll get the same answer if they do. He's not for sale."
Celtic paid £2m for Hooper when they signed him from Scunthorpe United in 2010.