Cahill was among 32 players picked in an expanded training squad, announced by coach Bert van Marwijk on Monday, but the veteran striker faces an anxious wait before knowing whether he will make the final cut for Russian Federation.
The Dutch coach, who will lead the Socceroos during the World Cup campagin before Graham Arnold takes over the reins, said Arzani's ability to turn the course of a game in the blink of an eye was behind his decision to pick the 19-year-old.
His addition to the squad leaves homegrown defenders, such as Josh Risdon and Bailey Wright, scrambling for a place when the squad is culled to 26 players next week.
"He's playing games, before March he didn't play every game".
"In tennis it's different, the players themselves can decide when you ask the video ref".
Van Marwijk identified right-back as a problem area for Australia, and has brought Karacic in to assess personally, on advice from his assistants. He doesn't play (at the moment), maybe five or 10 or 20 minutes.
Coach Bert Van Marwijk revealed 19-year-old Melbourne City flyer Arzani and 21-year-old Croatian based right-back Karacic would be part of the initial squad for Russian Federation at a press conference in Sydney on Monday. What I already said he is playing in the under-21 [national team] in Croatia.
"He also has an Australian passport and its possible for us to nominate him", van Marwijk said.More news: NASA's newest Mars lander to study quakes on Red Planet
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"He is a special case, he's special in everything, he's 38, he's probably one of the best players ever for Australia", he added. He's not 19 like Arzani, he's twice as old. All 26 players from Van Marwijk's first selection have been retained.
"I should be lying if I didn't say that maybe in head are 26 or 27 players", van Marwijk said. If he is selected for Australia and plays he will not be able to return to play for Croatia again.
"He plays only right-back and we don't have a lot of players in that position.it's a position we discuss a lot".
Cahill is now serving a suspension with Millwall and has played just 63 minutes over 10 games for the English Championship side since leaving the A-League in search of more game time.
"In those few weeks it must be very clear for the players what we want (at the 2018 FIFA World Cup)".
The Socceroos face France, Denmark and Peru in Group C at the June 14-July 15 finals.
Australia will play two friendlies against Hungary and the Czech Republic next month before taking on France in the first game of the World Cup on June 16.