Celtic first-team coach John Kennedy has explained to The Glasgow Evening Times why Jozo Simunovic did not train with the rest of the squad on Monday.
The Scottish Premiership champions started pre-season training on Monday.
The Bhoys have Champions League qualifiers coming up before they get into league action in Scotland.
Simunovic is expected to be an important player for Celtic during the 2016-17 campaign.
The 21-year-old central defender joined the Hoops from Croatian club Dinamo Zagreb in the summer of 2015.
According to The Glasgow Evening, the Glasgow giants paid just over £5 million as transfer fee in order to secure the services of the Croatia Under-21 international.
The centre-back has not played a competitive game since the end of January due to a knee problem.
Belgium international defender Dedryck Boyata is also out at the moment with a hamstring injury.
Celtic first-team coach Kennedy has revealed that both Simunovic and Boyata missed training on Monday due to their respective injury concerns.
“Both never trained today,” Kennedy told The Glasgow Evening Times about Celtic training on Monday.
“Jozo has a bit of work to do and Dedryck was scanned. Hopefully he’s not too bad. We’ll monitor them over a few weeks.”
Celtic will be aiming to win the Scottish Premiership title yet again next season, and will also be looking to progress to the group stage of the Champions League during the 2016-17 campaign.