Foley played his first match of the season for the Sydney team against Melbourne Rebels on Friday after missing four games following a pre-season head-knock but concussion symptoms returned after the 32-25 away win.
The team said in a statement on Tuesday that Foley was in doubt to play New Zealand's Canterbury Crusaders this weekend.
"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.
"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."
The Waratahs tried out different midfield combinations at training on Tuesday and David Horitz is a candidate to take the number 10 shirt.
"(Horwitz) played well last week. He stepped up into first receiver a lot, probably touched the ball more times than Bernard did," assistant coach Chris Malone said.
"Good growth from Dave and he's really come along in terms of being in the squad."
Foley's concussion woes will also be a concern for Wallabies coach Michael Cheika in the leadup to the June internationals against Fiji, Scotland and Italy.
"There's enough scary articles and different things out there and productions and film around concussion, and it's an issue today that it didn't used to be," Malone said.
"Therefore, from a coaching perspective, it's (only) a game at the end of the day. It's not worth risking the rest of someone's life for a game."
(Reporting by Ian Ransom; Editing by John O'Brien)
