Given the amount of talent the All Blacks have lost among their midfield ranks, though, Ngatai should still see himself rewarded with a spot in the 32-man squad, especially since Rennie was confident his key man's concussion had almost cleared.
"He's improved but just not quickly enough," the coach said. "We're really cautious around all the concussions, but I'm pretty confident that if we had a game next week he'd be playing."
Ngatai's absence means the impressive pairing of Seta Tamanivalu and Anton Lienert-Brown will continue the encouraging partnership they formed in last weekend's win over the Rebels, with Rennie opting for only one injury-enforced change to his starting XV.
Canterbury loose forward Tom Sanders comes into the side in place of Michael Leitch, with the Japanese captain today having an operation to repair the thumb he dislocated while helping the Chiefs back to the top of the table.
With Leitch facing an eight-to-10 week recovery period, Rennie was hopeful the No 8 could recover in time to feature in the playoffs - should the Chiefs punch their ticket in the final three games after the international break - but will in the meantime rely on Sanders and Maama Vaipulu. And with the latter last weekend suffering a head knock of his own while playing club rugby, Sanders has received the first crack.
"Tom's been really good," Rennie said. "He's had a couple of injuries and a concussion that took him out for a few weeks. But until maybe the last couple of weeks, every time he's been available he's started. He's got a lot of footy under his belt and he's learning lots, so we'll need a big one out of him."
Chiefs team to play the Waratahs in Sydney on Friday night:
Damian McKenzie, Toni Pulu, Seta Tamanivalu, Anton Lienert-Brown, James Lowe, Aaron Cruden (c), Brad Weber, Tom Sanders, Sam Cane (c), Taleni Seu, Brodie Retallick, Dominic Bird, Hiroshi Yamashita, Nathan Harris, Mitchell Graham.
Reserves: Hika Elliot, Kane Hames, Siegfried Fisi'ihoi, Michael Allardice, Lachlan Boshier, Kayne Hammington, Andrew Horrell, Sam Vaka.