Celtic have confirmed that Emilio Izaguirre will miss up to six months of the season after fracturing his ankle against Aberdeen on Sunday.
The Honduran left-back, 25, was hospitalised after leaving the field just 15 minutes into Sunday's 1-0 win at Pittodrie.
Scotland's player of the year went down after a challenge from Dons winger Peter Pawlett.
Manager Neil Lennon confirmed that the player had fractured his right fibula.
"It's a huge blow for us to lose him and we don't know how long he is going to be out for," Lennon told BBC Radio Scotland.
"He is being assessed at the minute and we'll know whether he requires surgery and, if he does, he will be out for three months anyway."
Lennon held his head in his hands when he was told the initial diagnosis that Izaguirre had suffered a suspected fracture.
"You can always tell by a player's reaction and Emilio is not a player to feign injury," added Lennon.
"So the concern was immediate and he was taken into the dressing-room and looked at and the word came out it was a fracture."
Lennon was already missing defenders Thomas Rogne, Glenn Loovens and Mark Wilson through injury, although the latter is due back from a groin problem for next week's Scottish Premier League meeting with Dundee United.
Scotland right-back Wilson has at times covered on the other flank, but Charlie Mulgrew, who replaced Izaguirre at Pittodrie, is the only experienced left-sided defender in Lennon's squad.
"We might have to look for another left-back," added Lennon.
Meanwhile, former Celtic striker Dion Dublin believes Izaguirre will have to take time to recover, both physically and emotionally, from his leg break.
Dublin, who suffered a broken leg and broken neck during a 23-year career, told BBC Scotland: "He'll be in pain, mentally as well as physically, and he won't want to explain that time after time.
"So, having been there, I would would suggest he is left for four or five days until he comes to terms with it but it's hard, it's really hard."