The inside centre was taken off during the Wallabies 23-21 first Test loss to the British and Irish Lions last Saturday in Brisbane and ruled out of the rest of the series.
Medical checks on Tuesday showed the 25-year-old had sustained a recurrence of a fracture of a bone in his neck that originally happened last November against France on Australia's northern hemisphere tour.
The injury means he will miss the remainder of the Super Rugby season with the ACT Brumbies, Australia's Rugby Championship campaign against South Africa, New Zealand and Argentina and their end-of-season tour to Europe.
"It's unfortunate that having completed the healing process from last year's injury, that it has happened again," Wallabies doctor Warren McDonald said.
"The fracture that has partially opened up is expected to repair itself and strengthen.
"This does not mean that Pat will not be able to return to the rugby field and be able to feel comfortable taking contact.
"The specialist advice is that a return is viable pending the full recovery and strengthening of the neck."
McCabe will have to spend at least the next six weeks in a brace while the injury stabilises before further assessment, the ARU added.
"It was encouraging to hear the specialists being positive both about the recovery process but also the possibility that I could be able to return to the playing field," McCabe said.
"It is human nature to think the worst but I'm in good hands medically. We'll see how the rehabilitation goes and take it from there.
"I'd obviously like to get back into it, but I'll be taking no chances with my health either."
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