Forster has forged a reputation as one of the best young keepers around after catching the eye during the last two seasons in the UEFA Champions League.
Lennon is a huge fan of the former Newcastle youngster and believes it is inevitable that Forster will be a target for the biggest clubs.
"It's going to be impossible to keep Fraser the way he's playing," he told PLZ Soccer's radio show. "Inevitably, I think a huge club will come in and make a huge bid for him.
"Sometimes it is very frustrating as a manager, as you are trying to build something, you are trying to progress the club but it is a huge compliment about what we have done over the past two or three years and it's put the club on the map again.
"People might say I am biased but over the last two seasons in the Champions League he has proved at the highest level he's a fantastic goalkeeper and he's still only 24.
"For a keeper who is still so young. He's got so much more to come.
"The way he's playing at the minute and the fact he's knocking on the door to be England's No 1 playing in the Scottish league, that speaks volumes for his talent."
Lennon is keen to keep the nucleus of his squad as he aims to build on their success of the last couple of seasons.
"We have a core - Brown, Ledley, Commons, Samaras, Lustig and Ambrose has just signed a new contract," added Lennon.
"It is important to keep that core together while one or two individuals may move on."