Celtic defender Daniel Majstorovic has ruptured a cruciate ligament during a training session with Sweden, his club have confirmed.
The 34-year-old, who was preparing for a friendly against Croatia in Zagreb when he suffered the injury in his left knee, will now miss the rest of the season.
A Celtic statement read: "Daniel will stay in Sweden for the immediate future and will have an operation there soon."
It continued: "Celtic´s medical team will be in close liaison with the Swedish FA medical team to ensure that Daniel receives the best of care. Unfortunately he will be out long-term."
Majstorovic will miss Celtic's attempts to secure a domestic treble and also Euro 2012, where Sweden face England, France and co-hosts Ukraine in the group stage.
The Celtic Park future of Majstorovic, who joined from AEK Athens in August 2010, is now uncertain as the centre-back is out of contract in the summer.
Manager Neil Lennon said in early December it would be difficult not to offer Majstorovic a new deal if he maintained his form, which was comfortably the best of his Celtic career.
Majstorovic soon said there was "no panic" over a new deal as he concentrated on Celtic's season and Euro 2012, but he then suffered a fractured eye socket in a heavy aerial challenge with St Johnstone midfielder David Robertson on December 18.
The former Malmo, FC Twente and Basle player was out for almost eight weeks before returning for Clydesdale Bank Premier League victories over Hearts and Inverness this month.
Majstorovic has been on the bench in recent weeks with Thomas Rogne and Kelvin Wilson returning to fitness and form in central defence. Charlie Mulgrew has also played there recently, while Glenn Loovens is pushing for a comeback after more than two months out injured.