Ireland flanker Chris Henry will be out of rugby for an unspecified period of time after tests detected a temporary blockage of a small blood vessel in his brain.
The openside had to withdraw from Ireland's international against South Africa on the morning of the game.
Henry was thought to have suffered a viral illness after feeling some weakness. On closer inspection, the blockage was uncovered by tests.
He has returned home where he will undergo further investigation and come under the guidance of specialist opinion.
This could open the door for Rhys Ruddock to stay at seven or swap flanks with Peter O'Mahony.
However, coach Joe Schmidt could decide a better presence on the floor from someone like Dominic Ryan or Tommy O'Donnell is required to counter Wallaby captain Michael Hooper.
On a more positive note, Rory Best is available to challenge Seán Cronin and Richardt Strauss for selection and will train fully this week.
The Ulster hooker will have his work cut out for him to unsettle the incumbent Cronin, although his superior scrummaging makes him 50-50 to start ahead of the two Leinster men.
There is a more cautious opinion on the bad bone-bruising to Jared Payne's foot.
The Ulster centre will not want to miss out on Australia for obvious reasons of his New Zealand heritage and the fact that Gordon D'Arcy is ready, willing and able to hold the inside centre berth, releasing Robbie Henshaw to thirteen.
Payne put to bed any doubts about his physicality for thirteen with a thoroughly professional shift outside Henshaw against South Africa.
Leinster lock Mike McCarthy suffered what has been diagnosed as concussion in the later stages of the comfortable victory over Georgia on Sunday.
He will follows the return-to-play protocols and is unlikely to factor on Saturday.