A lot had gone wrong for Leckie since his heroics at the Qatar World Cup in 2022, largely due to problematic hamstrings.
Before facing Mexico on Saturday, Melbourne City star Leckie last played for Australia against Palestine back in June 2024 and had never featured under Popovic.
He had hip surgery in December to alleviate his hamstring issues, before a monster A-League Men elimination final got Leckie into a training camp to push for selection.
"A couple of months ago, god knows what he was thinking in terms of his football with all the injuries that he's had," Popovic said.
After the Socceroos' first training session at their California base camp, Leckie said: "It's good to be here. I spoke to you guys over the past few months, past year about the frustrations of injury and and those types of things.
"It got to a point where I didn't think it was possible to be here.
"For me, the first point was to just try to get back fit and playing, so rather than even considering the national team, the goal was to get back fit.
"But I got back for just the end of the season, and I was fortunate that the boss gave me an opportunity to be here and prove myself.
"It's been three, four weeks now that I've been here, and I think that was super-important for me to come in early and get myself in a good physical spot and very happy that it was enough to make the final squad."
Leckie started against Mexico, built into that game and played 67 minutes.
He is confident he is in the best shape he can be right now, and will only keep building.
Leckie and Maty Ryan also join Tim Cahill and Mark Milligan as the only four-time World Cup Socceroos.
"It's quite unbelievable to be in the same position as players like them," Leckie said.
"It's something that I probably can look back on in the future and be super proud of, but there's still a long way to go."
Previously an explosive winger, Leckie has more recently played as a holding midfielder for City and an attacking midfielder/inverted winger with the Socceroos.
The 35-year-old is a plug-and-play option, with Popovic saying "you can basically show him a video and he would know what to do."
"In training I've done a bit of both over the past month, so that's obviously completely up to the boss," Leckie said.
"For me, I don't really have a preference. I like both positions in different ways, and I understand the responsibility of both.
"So, for me, wherever he thinks I can help the team, I'll play there."
