Brendan Rodgers was unable to explain Dedryck Boyata's absence from the Celtic team that crashed out of the Champions League on Tuesday.
Celtic lost 3-2 on aggregate to AEK Athens to miss out on a place in the play-off round of UEFA's lucrative continental club competition.
Rodgers insists Boyata is physically able to feature for the team, but the defender, who made three appearances for Belgium at the World Cup, did not make himself available for the game amid rumours the 27-year-old is seeking a move away from Celtic Park.
"He was fit," Rodgers said.
"We were missing our two best centre halves, unfortunately. My concentration until I get back is only with the players who were here. But yeah, Dedryck would have been fit to play.
"Is that frustrating? Yeah because he's been with us over the course of our journey for a couple of years.
"We got to this position and were without someone of his quality. Having a World Cup semi-finalist in our team would certainly have helped.
"But he wasn't here."
Earlier, Boyata released a video on Instagram showing him receiving treatment, accompanied by a caption saying: "Unfortunately I could not play tomorrow because I'm not yet 100% operational."
