The 22-year-old impressed last season for the Edinburgh side, making 37 appearances in all competitions, scoring five goals and registering 10 assists.
As a result, Celtic and Ipswich Town have been linked with the Scotland international this summer, but Lennon has issued a hands-off warning to potential suitors.
Lennon told The Scotsman: "He's not for sale. It's going to take a huge bid from any club to take John away from us, £4-5m. He's still got a bit to do but already he is a quality player. He's a Scottish international.
"I have got a proven international who is 22. He's not even anywhere near his peak yet but we already know what a good all-round player he is. He's one of the best players in Scotland for his age."
Hibs have already lost star man Jason Cummings to Nottingham Forest this summer.