Suaalii will be sidelined for up to eight weeks after tearing a hamstring in the warm-up before last Friday night's 59-12 capitulation to the Hurricanes in Sydney.
After fading from finals contention last year when fellow Wallabies strike weapon Max Jorgensen suffered an ankle injury, McKellar stressed the importance of keeping his big guns on the park this campaign.
Now the coach will be without a player he once famously dubbed the team's "Ferrari" for at least two-thirds of the season.
"It's just the next man's got to step up," McKellar said on Wednesday when asked how he'd manage sans Suaalii.
"It's obviously disappointing to lose any player to injury, and to have 'Su' on the sidelines at this stage it'll be six to eight weeks and we'll see how that goes, but the next person's just got to step up and you've just got to get on with it."
McKellar clearly wasn't happy with how George Poolman performed as the code-hopper's late replacement on debut, dropping the youngster and naming Triston Reilly at No.13 to face the Queensland Reds on Saturday night.
Reilly has played chiefly on the wing for the Tahs, but has the coach's backing to step up and fill Suaalii's centre spot in Brisbane.
"He's trained there pre-season for us, so he was always an option to be a back-up 13," McKellar said.
"He's a very, very good defender for us and understands how we want to defend, and his execution of that is excellent. He's pretty sharp, he's played 13 at under-20s level as well, so, yeah, full confidence in Triston."
With Reilly shifting infield, former Australian under-20s star Sid Harvey will start on the wing as the Waratahs bid to rebound from their 40-point shellacking and back up their season-opening 36-12 victory over the Reds.
"He was probably one of the shining lights from last Friday," McKellar said.
"He's just got this air of class about him, Sid, both as a footballer and as a human being. He's got time, very composed, doesn't get rattled.
"Last year we sent him away for a different experience to play sevens and be in a professional environment, always with the intention of bringing him here, and since he's set foot in the door after the sevens experience, he's been very good.
"He's one of our best in the Super 'Oz' competition and continues to take every opportunity he's been presented with."
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