Celtic captain Scott Brown is facing up to three months out with a knee injury.
The midfielder returned for Saturday's 0-0 draw at home to Kilmarnock after missing four matches but was hurt late in the game.
Manager Ronny Deila is not certain if the Scotland skipper will require surgery after a scan on Monday revealed the damage.
"We don't know for sure yet," said Deila. "But it could be eight to 12 weeks."
Deila said that the setback had nothing to do with Brown's previous injury.
"He was unfortunate in a challenge at the end of the game," explained the Norwegian. "He twisted it and he has an injury in the outside ligament.
"We don't know if he needs an operation but we hope that is not necessary.
"It is always tough to lose your captain but we won a lot of games during the last period he was out.
"It gives the other players more responsibility and they will grow with that."