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."
