'That's the difference': Why 35-year-old Socceroos veteran is on the verge of fourth World Cup

Socceroos coach Tony Popovic has given a clear indication that Mathew Leckie could set for another World Cup appearance at 35 years of age after his 120-minute performance in the A-League elimination final.

Leckie played in the midfield for Melbourne City against Auckland FC after returning from a hip injury last month.

The veteran has subsequently been picked in a pre World Cup camp in Sarasota, Florida with the others being Mitch Duke, Harry Souttar, Nishan Velupillay, Hayden Matthews, Anthony Caceres, Brandon Borrello and Nick D'Agostino.

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Popovic explained that while he's excited about the young cohort of players he's starting to assemble, Leckie's performance over the weekend showed the value of experience.

"If you watch the game that was just played, Mathew Leckie was the best player on the park at 35, with no football under his belt. That, a young player can't do as yet," Popovic told reporters on Monday in his final Australian appearance before the event.

"That's the difference, and that's what you need at a World Cup. Now will he make the World Cup? That will come down to his body. But if you just look at the quality of what he could do at 35 with no football under his belt, a young player can't do that. That's just normal.

"[They] will maybe do it in time with experience and maturity and strength. They offer something different; they offer raw talent, the quality they have, you can see something is there but we're not talking about friendly games now. We're talking about the World Cup and I think he [Leckie] showed on the weekend the difference."

Leckie's first World Cup was in 2014, and 2026 would be his fourth.

Souttar has also been troubled by injury, having just returned from an extended layoff with a ruptured Achilles. The centre back played two matches for Leicester at the end of the season for 90 minutes and 88 minutes and scored a goal against Millwall in the first fixture.

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Popovic was full of praise as he welcomed him back to the Socceroos setup, saying the mental resilience of Souttar and Leckie can't be bought.

"If you watch those two games (from Souttar), there is presence, there is aura, there is a leader," he said.

"You can't get that in a young player overnight. That takes time, and he's done it at a World Cup when he played also underdone. They've shown the resilience and mentality that you need at a World Cup, because you need to be able to suffer, you need to be able to go through tough times, these boys have done that in their own personal journey … the World Cup on another level again to anything we've done so far."

Popovic will have to submit his 26-man squad for the World Cup by June 1 and has a fresh injury worry with midfielder Aiden O'Neill going down with an injury on Monday morning (AEST) for New York City.

O'Neill was substituted off in the 36th minute with New York City's coach Pascal Jansen confirming post-match has played with ankle issues for a long time.

Popovic didn't have fresh information as of Monday's press conference.

"We've had a look at the footage, and it's hard to determine how serious it is at the moment," he said. "We're awaiting further tests, and we should know more in the next 24 to 48 hours."

The Socceroos' next matches will be friendlies against Mexico and Switzerland, with the first group match against Turkey on June 14 (AEST).

In those friendlies, Popovic is hoping to trial how the hydration breaks will work, which have been implemented for the World Cup. Those breaks will for three minutes in each half.

Popovic has previously spoken about the initiative, conceding they probably aren't needed for his players since they'll be playing in a cooler climate on the west coast of the United States and in Vancouver.

The coach flagged in March that he may look to AFL clubs for advice on how to deal with quarters and confirmed on Monday that he'd "sort out" other codes for a "better understanding" of how that can be handled.

"You can see even in the basketball when they have a time out there's obviously a clear strategy and there's a play they plan for after the break," he said.

"We've been trying to understand what we can do before and after the break ... when everyone cools down, what can we do or what should we do? We'll maybe try and practice a few things against Mexico and Switzerland, I'm sure they'll want to do that too. It's unique for everyone, not just unique for Australia.

"It's good to get two friendly games to practice that and see how the players feel, get some feedback on how they feel after each break, what do they feel they need to get going again."