Lee Naylor, Gary Caldwell and Stephen McManus' performances are keeping fit-again Bobo Balde out of the side, according to Celtic boss Gordon Strachan.
The three defenders have grown into their roles for The Hoops, while Balde underwent surgery over the summer on a stomach muscle problem.
But the Guinea international was a surprise inclusion on the bench for the midweek UEFA Champions League defeat to Manchester United.
While Strachan is happy to have his experienced centre back ready to play, he insists the current three mainstays have refused to be dropped, despite playing with injuries.
"They won't let me! They said they are fit enough to play on," Strachan explained.
"The spectre of Bobo coming back is keeping them going.
"But I'm okay with them playing on and I don't know if I can rush Bobo back."
Balde was again on the bench for the Bhoys' 1-0 win over Dunfermline on Saturday, along with striker Derek Riordan.
Riordan has played just two minutes of competitive action since joining the club from Hibernian for £200,000 this summer.
While Kaiserslautern are rumoured to be interested, Strachan has reassured the 23-year-old it is the Scottish Premier League's ruling that three Under-21 players must be in the squad, rather than his ability that is keeping him out of the side.
"Derek is in my thoughts. He is a good player," Strachan stated.
"The problem for him is that Aiden McGeady was voted Young Player of the Month for August and Shaun Maloney was Player of the Year last year.
"But the Under-21 rule doesn't help Derek or Stephen Pearson.
"I have to put players on the bench who maybe don't deserve to be there."
Riordan could be given a chance for the Scottish League Cup tie at home to St Mirren on Tuesday.