Norwich and Queens Park Rangers are both battling to sign the striker who has turned down a new deal to stay at Parkhead.
Hooper is entering the final year of his contract and Celtic are resigned to selling him rather than risk letting him see out his deal.
The 25-year-old has failed to feature in Celtic's last two pre-season friendlies with a groin problem and he is unlikely to play in next week's UEFA Champions League qualifier at Cliftonville.
Lennon admits it will be difficult to keep Hooper with Norwich and QPR expected to come back with new bids for the former Scunthorpe man.
"It will be difficult to convince Gary to stay," said Lennon. "He has not featured much in pre-season and the games start in earnest for us on Wednesday night.
"So he'd have a bit of catching up to do and I don't know where his mind is at the minute.
"Has he played his last game for Celtic? Never say never but it's looking more and more that way."
Lennon is hoping to bolster his squad after losing Victor Wanyama to Southampton, feeling he needs three players if they are to qualify for the Champions League.
"I haven't spoken to Peter Lawwell yet about bringing someone in but we need two or three players if we're going to get back into the Champions League," added Lennon.