The manager said he is bracing himself for a "huge bid" for the goalkeeper, the England squad player having enhanced his reputation with another top-class display in Celtic's win over Ajax at Celtic Park.
Lennon said during the summer that the 25-year-old, who signed a four-year contract last year, was "not for sale at any price", but now concedes he faces a struggle to keep potential suitors at bay.
"I think it's going to be impossible to keep Fraser the way he is playing," said Lennon. "Inevitably, I feel a big club will come in and make a huge bid for him. Sometimes it's frustrating as a manager because you're trying to build something and trying to progress as a club but I think it's a huge compliment to what we've done here in the last two or three years.
"People might say I'm biased, but I think over the last two seasons he has proved at the highest level that he is a fantastic keeper. He is only 25 and that is so young for a goalkeeper, and he has just got so much more to come in terms of his natural development.
"But the way he is playing at the minute and the fact that he is knocking on the door to be England's No.1, and he is playing in the Scottish league, speaks volumes for his talent."
Lennon was obviously delighted with his side's 2-1 victory on Tuesday which keeps alive hopes of qualification from the group stages but admitted he did not realise how good it was until he watched a rerun of the game in the small hours.
The win moved Celtic up to third in their section ahead of the November 6 return in Amsterdam and, unable to sleep, Lennon studied the match from the comfort of his living room.
"I actually watched the game again and I was up to about three, four o'clock. It's difficult to sleep after games, particularly an evening game. Having watched the game, we played better than I initially thought we did so that pleased me more.
"I thought the team were excellent, they pressed them at every opportunity and they tried not to let them get into a rhythm. Sometimes you get a better view of it when you're sitting in the quiet of the living room and you're watching the game on your own.
"I also watched Man City at CSKA Moscow [the following night]. You're always looking to learn from other teams as well, how they play away from home and how they adapt. I'm just trying to take it all in, it was a massive win, not just for me but the club and the players. We could have been looking into oblivion if we got beat in terms of any qualifying."
Ajax enjoyed 59% of possession at Celtic Park and Frank De Boer's side also had more attempts at goal. But Lennon, who watched his side dominate AC Milan for long
spells in their painful defeat in the
San Siro in the opening game of the section, insists he will not lose any sleep over the finer details of the game.
"I think sometimes you can go overboard with the possession stats. Chelsea won three-nil in Schalke but I think they only had 41, 42% of the ball, but Jose Mourinho won't be too concerned about that. A lot of the possession Ajax did have was in there own half and it wasn't hurting us. It can be a bit misleading at times. When I look at the stats after a game, I'm not convinced they're entirely accurate anyway.
"This is the elite, they are the Dutch champions, Barcelona
are the Spanish champions and Milan have great calibre in this competition for years. These guys are tremendous football players, great athletes.
"The game has changed,
the game has evolved and you can't always play a British type
of game but I think it's important we play at a good tempo at home because it gets the support going and it suits us at home to play that way."
Lennon, whose side return
to domestic action against Partick Thistle at Firhill on Sunday afternoon, has sampled the electric atmosphere on Champions League nights as both a player and manager and admits it never ceases to amaze him. "It was a brilliant night for us, you always think these European nights at Celtic Park can't get any better but they just seem to get better with every game. The atmosphere, the energy the support brought lifted the team to produce what was a great performance in the end;
lets not beat about the bush,
Ajax are a top-class team.
"We have an unbelievable home record and it's important for me, as the manager, to try to maintain that record, and beating Ajax has reinforced that."