Celtic have confirmed that Beram Kayal will miss the rest of the season after the midfielder underwent ankle surgery.
The Israeli international was stretchered off following a challenge with Lee McCulloch during the 1-0 win over Rangers on 28 December.
The 23-year-old had surgery in London on Monday evening.
And a statement on the club website read: "Unfortunately, our worst fears have been realised and Beram will be out for at least four months."
Kayal joined Celtic from Maccabi Haifa in 2010 and in October signed a new contract to keep him at the club until 2015.
On Friday, manager Neil Lennon described the player's injury as a "huge blow" for the club.
"It's no coincidence that this run has coincided with Beram signing a new contract," he said.
"He is a pivotal player for us. But I will say one area we have real quality strength in depth is midfield. I've got plenty of cover in midfield.
"But no-one likes to lose a player for a concerted period of time. We have been unfortunate with these injuries."
Regarding the tackle between Kayal and McCulloch, Lennon said: "It was a good challenge between two committed players.
"The way his ankle has turned, he has got a bit of cartilage floating in there that might need attaching back in and there is ligament damage as well."