Celtic defender Daniel Majstorovic has undergone surgery on a fractured cheekbone and will be out of action for six weeks.
The Swedish international was hurt in a challenge on St Johnstone's David Robertson during Sunday's 2-0 win.
"The operation went well, but he now faces six weeks on the sidelines before he can return to training," read a statement on the club website.
At the club's media conference at Lennoxtown on Thursday, Lennon added: "We were worried that the initial injury could have been a three month period [out] but we got really good news on the condition of the injury and after the surgery we are looking at maybe four-to-six weeks.
"He is up walking about. We are going to give him a few days off and then he will do some cardiovascular work.
"Obviously he can't get involved in any physical contact. It's a blow to lose him but it could have been a lot worse."
The 34-year-old's current deal expires at the end of the season.
Following the victory at McDiarmid Park, Lennon said Majstorovic had been "immense for the last month or so".
The Celtic boss and St Johnstone counterpart Steve Lomas disagreed about the fairness of a challenge that left Robertson with suspected concussion.
"It was a fair hard challenge, and he got to the ball first, then Robertson and him collided completely by accident," said Lennon.
"It was one of those good hard professional challenges that you want from your centre-half, but he came off on the wrong side of it."
However, Lennon's former Northern Ireland team-mate, Lomas, questioned the defender's challenge on the half-hour mark.
"I haven't seen it again, but I thought the big centre-half knew he was 70-30 in favour," said the St Johnstone manager.
"It might have just been an honest collision, but I wouldn't bet my house on it.
"Maybe you don't get those decisions against Celtic."