Head coach Ange Postecoglou made the revelation ahead of the Saturday lunchtime kick-off against St Johnstone at McDiarmid Park, which Celtic won 2-1 thanks to Giorgos Giakoumakis’ dramatic last gasp winner.
The Celtic boss admitted that the full extent of the damage to McGregor’s knee is still to be confirmed, but after seeing a specialist on Friday, he confirmed that his skipper will not return before the November shutdown for the World Cup in Qatar.
It means the Scotland international will miss at least 10 fixtures including his side’s remaining three Champions League group stage fixtures.
Speaking to Sky Sports, Postecoglou said: “Yeah, initial reports suggest that he’ll probably be out until after the break at this stage.
"I haven’t got the final report but he’ll miss the next three or four weeks which will put him out until after the break.
"The players are well aware of it, we went through a period last year where he was missing, and nothing really changes for us. It's about sticking to our team principles, he's a massive loss on and off the field, but just like any other club - we just have to deal with it.
"I'm disappointed for Callum more than anything. He's very influential in the group, he wants to be involved all the time and his performances have been super, but we just have to deal with what's ahead of us."
Postecoglou made six changes to his side for the trip to Perth following the midweek defeat to RB Leipzig in the Champions League.
Greg Taylor was left out the squad with Argentine left-back Alexandro Bernabei handed his first Premiership start, while Mortiz Jenz missed out due to illness.
The Celtic boss added: “Jenz woke up ill this morning.
"Greg is fine and we’ve just gave him a rest because he’s had a lot of games and I’m keen to give Bernabei a run.”