Celtic defender Dedryck Boyata faces up to three months out with a knee injury.
The Belgium international - who established himself as a regular starter in the second half of last season - will miss Celtic's Champions League qualifying campaign after suffering medial ligament damage during a friendly draw with Slavia Prague on Tuesday.
Assistant manager Chris Davies said: "It looks like a pretty bad knee injury really. The timeline we are looking at at the minute is up to three months, but we're always optimistic and we will see what we can do."
Davies was speaking at Glasgow Airport as Celtic flew out to Dublin for today's friendly against Shamrock Rovers without the former Manchester City centre-back.
"It's really disappointing for Dedryck," he said. "Obviously he did really well in the last six months of last season and was in good form and got an international call-up and things like that.
"We are all disappointed for him but there's no doubt about the work he's got to do in rehab and come back even stronger and better."
Boyata's injury will pave the way for Jozo Simunovic and Erik Sviatchenko to line up in central defence when Celtic begin their Champions League qualifiers against Linfield in Belfast on July 14.
But Celtic do not have many experienced alternatives.
Manager Brendan Rodgers talked up left-back Kieran Tierney as a possible Scotland centre-back and the 20-year-old has played in a back three for club and country, as has right-back Mikael Lustig for Celtic.
Centre-back Jamie McCart is also in Celtic's pre-season training squad but the 20-year-old has only made one substitute appearance for the club, although he played regularly on loan for Inverness last season.