And the Celtic manager reckons the odds have swung against the Hoops skipper recovering without going under the knife.
Brown has not played since limping out of the first leg of the Champions League last 16 match against Juventus nine days ago with an adductor muscle problem.
He has been struggling with a hip condition all season, and this latest injury has exacerbated the situation.
Lennon revealed on Monday that Brown was going to be examined by a specialist, and since then he has been bracing himself for the news he does not want to hear. The Parkhead boss now seems resigned to losing him for a prolonged period and said: "There is no news yet, but I do think he will need surgery."
It will be a huge blow to Celtic if they have to go to Turin on March 6 without their inspirational skipper.
Celtic already need a miracle to reverse the 3-0 deficit from the first leg, and having as many of their top players available as possible would increase their slim chances.
But, there is better news on Georgios Samaras, who, so often this season has been a goalscorer in European away games.
The Greek striker has been out of action since pulling a hamstring in the SPL win against Kilmarnock on January 30.
But, he is now back in training, and could be given the chance to test his damaged thigh on Sunday when Celtic play Dundee.
Lennon believes Sami's running power, height and goal threat were all badly missed when Juventus came to Glasgow last midweek.
And the manager reckons he can make an important contribution when they go to the Juventus Stadium in a fortnight.
"Samaras is an amazing player, and he is a talent," said Lennon, having overseen another training session which saw the 27-year-old step up his rehab.
"He has a boundless amount of energy and just keeps going right to the final whistle. He gets stronger as games go on.
"Apart from Scott Brown, he has been at the club the longest, and he has been a real leader on and off the field."
Samaras's inconsistency was his problem in the first few seasons of his time at the club.
But Lennon believes the more mature figure has overcome this, and can now be relied upon to deliver, no matter how big the game.
He said: "I knew he was talented, but he was very hit and miss. We had a long chat one day and he has never really looked back ever since then.
"While I have shown a lot of faith in him, he has repaid it in bundles. He is now a top, top, European forward."