Forster has been hailed as one of the best young keepers around and is being tracked by several of Europe's top clubs.
"It's going to be impossible to keep Fraser the way he's playing," Lennon 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 has really 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 that 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."