Duncan concluded four years with the Socceroos after the 2018 World Cup and has seen a new generation of Australian players progress to the national team level.

"It was rewarding firstly working with great coaches in Ange Postecoglou and Bert van Maarwijk," Duncan told FTBL.

"(I've worked with) wonderful players such as Tim Cahill, Mile Jedinak, Mark Milligan and also an exceptional backroom staff such as Les Gelis and Fabian Ehrmann.

"Robbie Kruse and Mat Leckie are probably the best football athletes in the squad, they both have speed and endurance which is an ideal combination."

Among Socceroos greats over the last four years, Duncan has overseen the progression of players like Massimo Luongo, Trent Sainsbury and Arzani.

A few weeks ago Arzani's career hit a hurdle following a sudden and shocking ACL injury which ruled him our for the rest of the season and the 2019 Asian Cup.

The injury came 20 minutes into his debut off the bench for Celtic following constant back-and-forth dialogue by fans about his potential future at the club being in question as a result of a lack of game time.

Duncan has seen and treated countless injuries just like this one and knows not only the limitations it could place physically on the player but also mentally.

"Obviously the injury will have a psychologist effect and players move through a process similar to the stages of grief," Duncan said.

"Often I see players become more determined and resilient during this period.

"It is all individual but the normal comeback is at approximately nine months although 12 months has been the most successful in my experience. However, it must be monitored from an individual perspective with a holistic and integrated management program."

Arzani made his international debut against Czech Republic and scored in his second match against Hungary. His three substitute cameos at the World Cup launched him to stardom as a loan deal to Celtic came calling.

For some players ACL injuries have the capability of limiting player potential. Duncan believes Arzani has the mentality to overcome this serious hurdle in his career.

"Daniel is a strong character and will get the best possible care and support," Duncan said.

"He will be even more determined and it will give him an opportunity to work on many things to increase his ability to perform.

"Often in life it is in the hurdles we must overcome that we get the greatest gains. Daniel is very young and I'm sure if he has the desire this hurdle will only make him stronger.

"Daniel is a very good person and I found him to be very open to feedback and most enjoyable to work with.

"Definitely there is a psych component to rehabilitation and I'm sure he will get that support at his club and also through his family."