Aberdeen 0 Celtic 1: Celtic left-back Emilio Izaguirre suffered a broken leg during the club's Clydesdale Bank Premier League victory at Aberdeen.
The Honduran left-back was carried off on a stretcher after going down in agony following a sliding challenge from Peter Pawlett.
Lennon revealed tests had shown up a fracture with the club soon confirming Izaguirre had broken his fibula and that he will be out for four to six months.
The injury happened when Izaguirre fell awkwardly following the challenge early in the game, which Celtic won 1-0 courtesy of a 74th-minute goal from Republic international Anthony Stokes.
Lennon said: “It's a huge blow for us to lose him and we don't know how long he is going to be out for.
“He is being assessed at the minute and we'll know whether he requires surgery and, if he does, he will be out for three months anyway.”
Lennon held his head in his hands when he was told the initial diagnosis that Izaguirre had suffered a suspected fracture.
“You can always tell by a player's reaction and Emilio is not a player to feign injury,” Lennon said.
“So the concern was immediate and he was taken into the dressing room and looked at and the word came out it was a fracture. We might have to look for another left-back.”
Celtic are currently in the transfer market for a goalkeeper with a deal for Poland international Grzegorz Sandomierski, from Jagiellonia Bialystok, coming to fruition.
“I'm not sure if it's a done deal, I haven't spoken to Peter today, but we might be further down the line with that one,” Lennon said. “He's a goalkeeper we do like and we feel he has huge potential.”
Lennon felt his team overplayed in the final third before Stokes earned them victory.
Celtic struggled to create a clear opening before right winger Kris Commons dispossessed Richard Foster and set up Stokes to sweep home the only goal from 10 yards in the 74th minute.
The Celtic manager said: “We made it nervous for ourselves because we had so many chances and our decision-making on the final bit was poor.
“We worked the ball great, kept getting in between their midfield and back four and then we tried to walk the ball in the net at times.
“It's a bad habit to have and we have to get out of it. I said at half-time we needed to be patient, we would take 1-0 and that happened to be the case. But I felt our play deserved better, overall we were very, very good.”
The Ulsterman had just moved Stokes into the middle before his goal after bringing on James Forrest for Joe Ledley, but he was planning another change.
Lennon said: “We were thinking of taking Stokesy off, believe it or not, so it shows you how much I know, and maybe getting Paddy (McCourt) in a wider area.
“We worked the ball really well into the wider areas and just needed an eliminator to pick the right option, and Paddy is very good at that. Thankfully we got the goal before we made the substitution.”
l Motherwell went top of the Clydesdale Bank Premier League after Jamie Murphy scored a second-half winner against Hearts at Fir Park.