Celtic manager Ange Postecoglou expects to lose Callum McGregor for a significant period as he awaits an update on his captain's knee injury.
The Scotland midfielder was forced off in Wednesday's Champions League defeat by RB Leipzig.
Postecoglou said the 29-year-old was scheduled to see a specialist on Friday morning.
"It's obviously not just a short-term one but the extent of it, we'll have to wait and see," he said.
McGregor was going to miss Saturday's visit to St Johnstone through suspension but Celtic will also be without another central midfielder, David Turnbull, who is recovering from tonsillitis.
Oliver Abildgaard took over from the injured skipper in Germany as the Dane came on to make his debut.
"There's a reason Callum is the captain," said Postecoglou. "He's a fantastic player who contributes to the way we play our football. He is influential on and off the field so you can't just replace him with one person overnight.
"That just doesn't happen. It's about other people filling in the breach. We missed him for a few games last season, it's just something we have to cope with.
"I'm disappointed for Callum more than anything because he wants to be involved all the time. His performances have been super but we just have to deal with what's ahead of us.
"The one thing I know about Callum is whatever timeline they give him, he'll try and come back earlier. That's what happened last year with his cheek fracture. He got the mask on and played.
"He's been around long enough to know this is part of being a professional footballer. You're not always going to play 70 games a year. Injuries will happen. He's just been unlucky the other night. He's a professional on and off the field and he'll tackle his rehab as determined as anyone."