Bernard Foley's concussion symptoms have returned after the NSW Waratahs star played in their Super Rugby win over Melbourne on the weekend.
The Wallabies five-eighth is in doubt for the round-six clash with the Crusaders in Sydney on Sunday having missed the Waratahs' opening four games after suffering the head knock in pre-season.
The Waratahs said Foley had passed the required concussion tests before being selected to face the Rebels.
"Bernard's welfare is our priority right now and our medical team are taking an active treatment process to manage his symptoms," Waratahs coach Daryl Gibson said in a statement on Tuesday.
"We will follow the medical advice and he will undergo further testing and treatment prior to any decisions being made about him returning to playing."
Foley overcame the injury to star in the 32-25 win in Melbourne on Friday, when his head was tested early with a couple of big Rebels tackles before leading his side's second-half revival.
It was a crucial win to improve the Sydney-based side's win-loss record to 2-3 and keep their Super Rugby season alive.
In weeks leading up to the Rebels clash, Foley would some days wake up fine but feel unwell on others.
Similarly with training he would endure headaches after some sessions.