Sportal.co.nz 13.Sep.2013Getty Images
McKenzie dropped the selection bombshell on Wednesday in a desperate bid to turn things around after three-straight losses to start his Wallabies tenure.
Brumbies young-gun Nic White will make his run-on debut at the expense of 47-Test veteran Genia, who is widely regarded the best number nine in world rugby.
"Decisions in life as a leader, you get defined by the hard decisions, not by the easy ones," McKenzie said.
"Everyone can make easy decisions, the hard ones are the ones that make the difference."
"... Everyone knows Will's an outstanding player but so is Nic White.
"The one thing I've done is come into the Wallabies environment and as I said at the start, I'm meeting some guys for the first time, and he's one of the guys I knew not so much about.
"He's done everything that has impressed me and he's taken the little moments we've given him on the field and trained hard.
"In the end it's always going to be a competition for places, the Wallabies needs to be an elite team where it's competitive.
"We're looking for the best outcomes on the field and we haven't been getting those but we're going to keep searching."
McKenzie, who coached Genia for four seasons at the Reds, backed the 25-year-old to fight his way out of his current form slump.
"I've known Will for a long time and they're difficult conversations, they always are, but that's the nature of the business," he said.
"There's two sides to every selection decision, there's the guy going out and the guy coming in.
"Will's an outstanding player and he'll admit he probably hasn't been at the peak of his game of late.
"But he'll respond as he already has in training ... he pushes himself very hard and he'll continue to do that.
"What we want ultimately is a bunch of players in every position competing hard and giving us success."
White, who made his Test debut against New Zealand in Sydney last month, admits he was shocked when McKenzie broke the news he would be starting against the Pumas.
"It's a huge surprise," White, 22, said on Wednesday.
"Ewen tapped me on the shoulder at training and said 'you'll start this week'.
"It was just straight away excitement.
"I was pretty nervous before that training session, the bus ride was pretty fun to training but as soon as I got to training the realisation that I've got to get a job done this week (set in).
"I've been put in for a reason and just had to get straight into nutting out what I had to do."