After missing the Waratahs' first four games, Foley returned for the away win over the Melbourne Rebels last week but his symptoms reappeared after the match in a worrying sign for the Wallabies playmaker.
The Waratahs, who have two wins from their opening five fixtures, will throw debutant Mack Mason in at the deep end by giving the Australian under-20 international the number 10 shirt against the competition's in-form side.
Champions in 2014, the Waratahs have struggled badly this season and their hopes of getting back on track at the Sydney Football Stadium on Sunday were dealt a further blow with Wallabies centre Rob Horne sidelined with a hamstring strain.
Israel Folau, who proved ineffective in a midfield stint during the opening games of the season, has been shifted to centre to replace Horne, with Cam Clark starting at fullback.
Foley's relapse has raised questions about his selection for last week's game against the Rebels, prompting the Sydney team to defend its team doctor for giving the all-clear.
Foley said his relapse had "nothing to do with the game last week" but was unsure when he might return.
"It's a tough one. Ultimately, I think it is just time," Foley said on Sky Sports Radio.
"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 percent for a couple of weeks and it's still ongoing for me."
(Reporting by Ian Ransom; Editing by Peter Rutherford)
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