Chris Jullien has flown to Germany to find a solution to his back injury.
The defender hasn’t played since Sptember 12 away to Ross County with no timeline in place for his return.
Jullien was an unused substitute against St Mirren then sat out three matches before returning to bench duty against Sarajevo and St Johnstone.
The international break provided extra time to come up with a cure but after missing the Glasgow derby a specialist has been sought out.
Lennon told the Daily Record:
We’re missing Jullien because I thought he had a stellar season last year. He’s got a back issue that’s been rumbling on for a few weeks. He’s gone back to Germany to get some round the clock treatment.
Hopefully that will work. He’s had scans and he has been in a lot of discomfort but there isn’t much showing up on the scans.
With Hatem Abd Elhamed and Nir Bitton also missing out Stephen Welsh has played in Celtic’s last two matches to maintain a back three.
Tom Rogic and Ryan Christie’s availability at Aberdeen could see Celtic return to a four man defence with Bitton expected to be available for Thursday’s Europa League tie away to Lille.