Scott Brown is facing up to 10 weeks on the sidelines with a stress reaction in a metatarsal on his right foot.
The Celtic skipper will not now need surgery on his broken foot as first feared.
The club announced on Friday that they expected him to be out of action for at least six weeks.
A visit to a specialist on Wednesday allayed fears he would need surgery, which would have kept him out for around three months.
But a statement on the club's official website read: "However, with the extra rehabilitation work required, the midfielder is likely to be out for eight to 10 weeks."
Brown confessed on Monday that he was devastated with the news he would be out of action for the next couple of months saying: "I'm devastated. No player wants to sit out, especially me.
"I'm not one for sitting in the stands. I want to be involved. I'm not a great spectator.
"There are just so many important games coming up for Celtic and Scotland and I'm gutted I won't be able to play in them."
Israeli midfielder Beram Kayal is already out for a month at least, making Brown's loss another blow to manager Neil Lennon.