The Reds overcame a slow start to win 35-15 on Saturday at Suncorp Stadium, McReight finishing with his shoulder in a sling after falling awkwardly as he scored a fantastic second-half try.
Reds coach Les Kiss, who had planned to rest McReight for next week's game against the Highlanders in New Zealand, confirmed it was an AC joint injury but he's hopeful the issue will not be a long-term one.
The Reds, who are now 3-1 after a tough start that's included games in Perth and Christchurch, are already without one-time Wallabies captain and fellow backrower Liam Wright.
He'll see a specialist about his own recurring shoulder injury that saw him miss Saturday's game.
The side was also without lock Lukhan Salakaia-Loto, centre Josh Flook and fullback Jock Campbell, while hooker Matt Faessler left Saturday's game with a leg issue.
But captain McDermott said their depth, particularly in the backrow, had already been proven in 2024 when Harry Wilson broke his arm and Fraser McReight served a suspension.
"It's a challenge, but we saw the same thing kind of happen last year and Johnny Bryant stepped up massively," the scrumhalf said.
"Joe Brial coming in there (to replace McReight); we've got the depth and we back whoever steps in, because we saw last year those guys are bloody good players.
The fiery clash saw Wallabies teammates Andrew Kellaway and Wilson exchange words after the Waratahs fullback up-ended Test skipper Wilson in a crunching tackle and was yellow-carded.
That incident triggered the Reds' resurgence, their dominance summed up in a 20-metre driving maul try while a host of Waratahs stood and watched.
"It was a big moment; I think the whole game emotionally was a battle and we knew it would be," McDermott said.
The rolling maul try infuriated Waratahs coach Dan McKellar, who had a week earlier copped some flak for labelling the Reds "the best team in the comp".
"It's hard to stop a rolling maul if you stand there and watch it," he sneered, describing that effort as "border-line embarrassing".
McKellar lamented his team's "soft moments" and said he meant what he'd said about the Reds a week earlier.
"Everyone sort of thought that was a bit of tongue in cheek," he said.
"I was being honest; when they get their game going, they're dangerous, as good as anyone and they showed that."
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