Celtic captain Scott Brown faces up to ten weeks on the sidelines, as he requires surgery on his ankle injury.
Brown has been troubled with the problem all season, with the midfielder initially sustaining the injury while playing for Scotland.
The 26-year-old aggravated the injury during last month's Old Firm derby defeat to Rangers and he has been forced to miss the club's last four games.
The club were hoping a period of rest and recuperation would cure the problem, but they have now been forced to send him under the knife.
The surgery means Brown is unlikely to return until December at the earliest.
The news is a major blow for under-fire Celtic boss Neil Lennon, with the team currently trailing Rangers by ten points in the SPL and also struggling to progress in the Europa League after taking just one point from their first two games.