Bernard Foley has been ruled out of Waratahs' Super Rugby clash with Crusaders on Sunday as he continues to struggle with symptoms of concussion and is unsure when he will be able to return.
The Australia fly-half made his first appearance of the season against Rebels last Friday after a spell on the sidelines due to a blow to the head he took in a trial match against Highlanders.
However, he suffered a setback following his return and will not feature at Allianz Stadium this weekend.
Foley, who said he feared for his career as he endured a "scary" struggle to get back on the field, on Friday defended Waratahs' medical team for allowing him to play last week and is prepared to be patient in order to resolve the problems he has encountered.
"It's been a frustrating week, in terms of symptoms just re-occurring [but] nothing to do with the game last week," he told Sky Sports Radio.
"It's just sort of come back after the game."
He added: "It's a tough one. Ultimately, I think it is just time,
"With the nature of head knocks and concussion, no one really knows. It is just time and you know when you feel right, when you're back to normal.
"When it first happened, you think you probably play the next week and it just sort of lingers and you just don't feel 100 per cent for a couple of weeks and it's still ongoing for me."