But what interested him more was how the teen sensation responded the next week - and the fresh maturity behind it.
In Watford's 2-2 draw with Portsmouth, the winger was hooked after 35 minutes and upon reaching the bench, Irankunda punched his seat and threw a water bottle at the floor before storming down the tunnel.
"It's just part of experience as a footballer," Popovic said ahead of friendlies against Canada and the United States.
"He's experienced a lot in the last 12 months. He's getting better. He's maturing.
"... I think the most important part of that is not what happened there. It's what happened the week later.
"Because I watched the game after, they won 2-1 (against Oxford) and he probably played and moved as well as he has for the last couple of weeks.
"So there was a reaction, albeit I don't know what was said between the two, but he was very positive, very good in terms of his performance and he really contributed in how he played to help the team win 2-1.
"So I think that's the important part, not so much of what happened with getting taken off because that may not be the last time that that happens to him."
Irankunda had a sensational game against New Zealand last month, scoring a wonderful goal.
But it's increased resilience and ability to back up game after game - and after a "setback" - that delighted Popovic, who took a tough-love approach to the youngster earlier in his tenure.
"I watched the game. For me, it was important to watch the next game," he said.
"I believe 12 months ago, Nestory doesn't perform that way. So he's maturing. He's getting stronger physically and mentally."
The Socceroos have also welcomed talented playmaker Alex Robertson's decision to play for Australia, ending an almost two-year standoff.
Robertson has received his first call-up since October 2023 despite being yet to register any minutes at Cardiff City this season.
"What's changed? Alex has made himself available for the Socceroos," Popovic said.
"He wants to play for Australia and he informed us of that about a month ago. Since that time we've been monitoring him.
"... I just felt it was a good time to see where he is now.
"I know he obviously has no minutes under his belt. He has a situation at Cardiff which I need to understand a little bit better from him.
"But it's a chance to really see his current level, how he is in our environment and then, which we hope for all players, that they get game time, that we understand where his base level is without football.
"And then if he gets the football, which we anticipate he will, it gives him an opportunity to be selected for the World Cup."
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