Celtic captain Scott Brown is to undergo ankle surgery and will be out for up to 10 weeks, the club confirmed today.
The midfielder initially sustained the injury while playing for Scotland in August and has missed the last four games for the Hoops.
He will now have "a minor procedure" on the joint, handing another headache to Celtic boss Neil Lennon, whose side currently lie third in the SPL table, 10 points behind Rangers.
Brown twice underwent surgery on the affected right ankle in 2009 but he has insisted the latest injury is unrelated to his earlier issues.
The news will come as a bitter blow to Lennon, who is facing serious scrutiny following Celtic's disappointing run of form.
Celtic have ruled Brown out for eight to 10 weeks, the 26-year-old joining Kelvin Wilson and Emilio Izaguirre on the long-term injury list.
Wilson is expected to be out for a similar period with an Achilles injury while Izaguirre is not due back until December at the earliest.
Lennon was also without Beram Kayal, Cha Du-ri and Joe Ledley for Sunday's 2-0 defeat against Hearts, their fifth in 10 games, although Ledley expects to return from his groin problem when Celtic visit Kilmarnock on October 15.
Right-back Mark Wilson, who has himself missed several games through injury this season, believes the lack of consistency in selection has been a major factor in the team's recent struggles in defence.
"It would be nice to get a settled back four because I think that's where a lot of our success came from last season," Wilson told the Celtic View.
"We were solid and kept a lot of clean sheets and it would be nice to get back to that.
"The changes have been happening across the defence, mainly in the middle but in my position too.
"There are three right-backs and circumstances have had the manager using them all rather than just sticking to a certain one.
"It's good there are three of us who can go in there and play comfortably but on the flip side it would be nice to get a run of games because with a run of games comes confidence and self-belief.
"It helps you settle into the team and play better. I think you can see that in other positions because some of the other boys, like Ki (Sung-yueng), have had a run of games and are settled now."