Waratahs will take no risks with Bernard Foley after the Australia fly-half experienced more symptoms of concussion following his comeback against Rebels last Friday.
Foley came through his first Super Rugby appearance of the season at AAMI Park, having missed the opening five weeks due to a blow to the head sustained in a trial match against Highlanders.
The playmaker, who said he feared his career may be over during a "scary" struggle to get back on the field before facing Reds, has since reported a setback and could face another spell on the sidelines.
Waratahs assistant coach Chris Malone said Foley will be given as long as he needs to recover.
"It's unclear exactly why those symptoms may have returned," Malone said.
"With it being recurring and why a couple of symptoms have returned, we just have to investigate why before we even worry about him playing rugby,
"The great beauty of our medical staff in particular is ensuring that guys aren't going out to play when they shouldn't be.
"We have to just ensure that he's looked after,
"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.
"Therefore, from a coaching perspective, it's a game at the end of the day. It's not worth risking the rest of someone's life for a game.
"So that's why we'll follow the protocol that we have to follow. I'm fully confident that in time, Nard will be sweet. And that may well be tomorrow. That may well be the day after that.
"So we've just got to play it by ear and ensure that we look after him."