McCabe returned to Australia in a neck brace and will have scans in Sydney on Monday or Tuesday after feeling similar symptoms to the first two times he broke his neck in the Wallabies' 51-20 Bledisloe Cup loss to New Zealand.

The Brumbies and Wallabies are holding hope there's no break and it's a similar incident to earlier this year, when it was feared McCabe would never play again into the best form of his career.

But it's a heartbreaking blow for one of world rugby's toughest players, having defied the odds to come back from two career-threatening injuries and fearing his career was over when he suffered another knock earlier this year.

It's understood McCabe was told a third break in the same spot in his neck would almost certainly force a premature end to his playing days.

McCabe, 26, has played just seven Tests since November 2012 and injured his neck in three of them.

But Brumbies coach Stephen Larkham said McCabe had already proved he had the mental toughness to bounce back from confidence-shattering injuries.

"It's devastating news for Patty. He'll get more scans and until that we can't look too far forward," Larkham said.

"We hope he will be OK and we'll assess it later. If there is a little crack, it would be difficult mentally to return. But Pat has already done it twice and he's done it successfully.

"He's a very strong character. The boys joked in the pre-season how tough he is, and he is genuinely tough.

"If anyone could come back, he could. He'll make any decision not just on rugby, but on life as well."

Brumbies winger Joseph Tomane looms as a potential Test replacement for McCabe for the next Wallabies clash against South Africa on September 6.

But the focus on Sunday was on McCabe's future. McCabe was in career-best form for the Brumbies this year, launching an undeniable case for a recall to the Wallabies set up.

He broke his neck on the spring tour in November 2012 and then again against the British and Irish Lions this year.

In the Brumbies' clash against the Western Force in March, McCabe thought he had broken it again and was taken to hospital in an ambulance thinking he would never play again.

Somehow he pushed any mental doubts aside and bounced back the following week with an outstanding performance against the Wellington Hurricanes, prompting ACT skipper Ben Mowen to dub him Super Rugby's version of "Chuck Norris".

Humble McCabe downplayed his comeback and continued his hard-nosed approach to the game to get his career back on track.

But a seemingly innocuous incident could be his downfall in the Wallabies Eden Park thrashing.

"He just got a knock and felt it on the top of his head," said Wallabies coach Ewen McKenzie.

"When you have these injuries you get a feeling for the symptoms and he felt there were similarities so he came straight off.

"He had a scan last night, there's no risk or anything, the brace is purely precautionary, but it shows on the scan that there might be a recurrence of the same injury."

The 23-Test back made his return to international rugby against France this year after a superb season for the Brumbies.

"This was his best year yet, he was outstanding," Larkham said.

McCabe said in May that he fell out of love with rugby after injury riddled years in 2012 and 2013.

"I really didn't enjoy myself at all last year and to be here loving footy again … it's a very nice feeling," McCabe said.

"Last year I never really felt like I was contributing or that I was overly confident in my ability to play well for the team."

McCabe drew confidence from every game he played, including the knock on his neck in the Force game which he thought would end his career.

"That knock [has] actually given me confidence," McCabe said.

"Once I got that big knock, had a scare and it was fine, it gave me confidence to know that I could survive. I do think where I'd be now if I'd broken my neck again.

"When I was in the ambulance I thought I'd played my last game. That was it. It was a scary and daunting proposition.

"But I've found a lot more enjoyment in the game this year."