The Australian was left ruining the “story of our season” over injury losses after McGregor was forced off approaching the interval holding a poultice to the side of his face as a result of a clash of heads with Adam King thumping into his opponent as the pair challenged for a high ball.
In the second period, Liel Abada and Yosuke Ideguchi were also casualties of a bruising encounter, but the fears over McGregor were the most palpable - even if Celtic manager point-blank refused to speculate on timescales, or the nature of the problem.
There seems little chance he will be available in the next few weeks - ruling him out of Hearts away on Wednesday and, crucially, the top-of-the-table derby at home to Rangers on February 2. The 56-year-old became irritated, though, when asked specifically about McGregor’s chances of returning for that potentially pivotal confrontation.
“Cal's a significant injury. It's a facial injury, I don't know what it is but he's in not a great shape. It's not going to be a short-term, it looked significant,” said a dismayed Postecoglou. "That's been our season. There's not been a weekend where I have everyone available, maybe it's still to come. We just have to get on with it.
"We've got a game Wednesday night, you guys can talk about the fictitious future that might arise, but the game on Wednesday night is all we are focused on and he won't play Wednesday night, which is a blow. I am not a doctor. There's a point I can't go beyond. It's a significant injury.”
It took time - more than two miinutes - for referee Don Robertson to allow Celtic’s medical team to enter the pitch to treat McGregor and, though, Postecoglou rarely makes reference to the actions of officials the delay in allowing the midfielder to be attended to for what immediately appeared a worrying situation led to the Celtic manager breaking with convention.
"The refs are out there... From what I understand he wasn't in great shape right from the start,” said Postecoglou, who suggested Abada and Ideguchi would shake off their knocks before midweek. “That's the disappointing thing.”