Waratahs fly-half Bernard Foley has been ruled out of Saturday's clash with the Brumbies as he is still recovering from a head knock sustained during the pre-season.
The 27-year-old was set to make his first appearance in this year's tournament but was ruled out when he failed to meet concussion protocols after the Waratahs' captain's run in Sydney on Friday.
"Bernard isn't quite 100 per cent and still experiencing some symptoms," Waratahs head coach Daryl Gibson told AAP.
"He has trained all week and completed his cognitive tests. We have seen improvements but he still has some symptoms lingering.
"It's frustrating for Bernard being so very close – he more than anyone wants to be back out on the field.
"But we won't want to push him so we'll give him more time and ensure we take all the necessary precautions to get him back to 100 per cent."
Bryce Hegarty, who was initially bracketed with Foley will start at fly-half for the fourth game in a row while David Horwitz will start on the replacements bench.
Waratahs (revised): 15 Andrew Kellaway, 14 Reece Robinson, 13 Israel Folau, 12 Irae Simone, 11 Rob Horne, 10 Bryce Hegarty, 9 Nick Phipps, 8 Jed Holloway, 7 Michael Hooper (c), 6 Jack Dempsey, 5 Will Skelton, 4 Dean Mumm, 3 Sekope Kepu, 2 Tolu Latu, 1 Tom Robertson
Replacements: 16 Hugh Roach, 17 Paddy Ryan, 18 David Lolohea, 19 David McDuling, 20 Ned Hanigan, 21 Michael Wells, 22 Jake Gordon, 23 David Horwitz
Date: Saturday, March 18
Venue: Allianz Stadium, Sydney
Kick-off: 19:45 local (8:45 GMT)
Referee: Glen Jackson (New Zealand)
Assistant Referees: Nic Berry (Australia), William Houston (Australia)
TMO: Ian Smith (Australia)