Duncan worked one-on-one with Arzani ahead of the 2018 FIFA World Cup in Russia and believes the 22-year-old - currently on loan but out of favour at FC Utrecht - must dredge deep and follow benchmarks of endurance and intensity set by the likes of Mathew Leckie, Robbie Kruse, Mile Jedinak and Tim Cahill to get the best from himself.
Not blessed with the freakish aerobic attributes of a Leckie or Kruse, Duncan contends that Arzani is “at a crossroads” in his journey to deliver on the stratospheric expectations which have followed him since his breakthrough at Melbourne City and subsequent moves to Manchester City and Celtic.
“Daniel has incredible technical ability and is a very intelligent person and an intelligent player,” said Duncan.
“Right now, though, he’s at a crossroads in terms of his future.
“If he can get his physiology up to the right standard, then anything can happen for him.
“But that’s the question - whether he can achieve that.
“The thing that separates greatness from everything else is desire.”
It’s not necessarily the distance players cover in games that’s telling, says Duncan, it’s the “high intensity, and explosive actions” which ideally comprise up to 14 per cent of, for example, the 12.5km Leckie might cover in Socceroos game.
These are figures attacker Arzani, even before his ACL injury on debut for Celtic well over two years ago, can only aspire to.
“It’s all about high-speed metres at the top level, especially for the wide players,” explained Duncan, who recently teamed up with Football NSW.
“You look at Robbie Kruse, he copped a lot (of abuse from fans) playing for Australia, but they didn’t have a clue how much work that kid put in to make a difference to the team.
“Both he and Leckie were incredible workers for the cause.
“With Kruse, you have to remember that (like Arzani) he suffered his share of serious injuries.
“You need to look at these players holistically as a person, both psychologically and physiologically.
“Robbie just needed to be treated well. I never understood the criticism he got because I knew from the data how much he was putting into the games.
“Why does Mile Jedinak have a long career? He’s big and strong, is physically good and works from morning till night to get himself in the best shape, just like Timmy.
“Both were injury resilient, and that gives you a long career.
“I’m very fond of Timmy and was proud to see him go to a fourth World Cup.
“The most disappointing thing was that he retired because if there’s anybody who could’ve played until the age of 45, it would have been to be a Tim Cahill.”
Australia’s high-performance chief for four years and on Iran’s staff under Carlos Queiroz at the 2019 AFC Asian Cup, Duncan claims the brute physicality of previous Socceroos generations went out the window when Dutchman Han Berger arrived as technical director and threw all his eggs in the technical development basket.
“We’re not the Australia we used to be where grew up playing multiple sports and were great athletes,” he added.
“We don’t have that anymore - we have kids who play lots of football, and they might be technically good, but they can’t move.
“We’re very confused, and a lot of our coaches are very confused unless they’ve worked at a high level overseas.
“The reality is that if you don’t have a strong physical component to your game, you can’t be successful.
“You’ll spend one week in Europe, and you’ll be sent back. Remember the golden generation - they all had a great physical presence about them.”
Once to have recently jolted Duncan with his physical prowess is Seattle Sounders’ Brad Smith, who was nurtured at Liverpool before spells at Bournemouth and Cardiff City.
“He’s like a Formula One machine with his pace, but there’s also the work needed to stop players like him breaking down (an issue for the wingback thus far in his career).
“In a game up to 14 per cent of his total metres would be at high speed - that’s world-class physiology—the same as Leckie.
“We don’t have too many players like that, and you need them to be successful.”
Enigmatic Celtic attacking midfielder Tom Rogic is another who Duncan feels hasn’t mined the full depths of his physiology.
“If you cover the technical, the tactical, the psychological and the physical then you’re a superstar,” he explained.
“If one of those is out, then you’re still going to get places.
“I’ve seen Tom play really well, particularly in the World Cup playoff against Honduras in Sydney (in November 2017).
“Sometimes when people are really technically gifted, it takes away from them working the other side of their game.
“Rogic, though, is a player who can win you a game with something special, and you can work with that, even if he doesn’t maybe do a job defensively.”