"He's 38. He is maybe one of the best players to ever play for Australia", Van Marwijk said.
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.
Karacic's father is Australian but he hasn't played in Australia.
Another three players will then suffer in Van Marwijk's final cut.
Striker Jamie McLaren's recent goal-scoring exploits in Scotland won him a recall alongside goalkeeper Mitchell Langerak, Alex Gersbach and Apostolos Giannou.More news: Stock with Surprising Moves in Wall Street: Fitbit Inc (FIT)
The NK Lokomotiva rightback plies his trade in the Croatian first division - Prva HNL - for the sixth-placed side from Zagreb.
"Nobody told me not to nominate him because of this, but I hope that when we are nominating him he will play for Australia", he said.
"He is young and must learn a lot but he is a player who can make a difference".
"He's a very good striker (but) he doesn't play and I have to take a decision on whether to take him to the world championships".
"I have to make a choice [by selection deadline] about the strikers".More news: Israel Says Better to Confront Iran Sooner Rather Than Later
"He is not 19 like Arzani - he's twice as old - but is also a player who can make a difference".
There were few shocks in the makeup of the squad with van Marwijk sticking with the experienced core of players who guided the Socceroos through the long qualifying phase although the Dutchman did include one surprise by picking uncapped defender Fran Karacic.
"We have travelled extensively and watched many, many matches because we want to get as much information on as many players as possible", Van Marwijk explained. "But we have since seen other players so there will be a few new names in the squad".
Australia assemble in Turkey for the start of their pre-tournament training camp from May 19. A decision on the final 23 made must be made by 4 June.
Australia face France, Denmark and Peru in Group C. They will play friendlies against Czech Republic and Hungary before their curtain-raiser in Russian Federation. "In those few weeks it must be very clear for the players what we want [at Russian Federation 2018]". There are two remaining friendlies to play before the team head to Russian Federation - against the Czech Republic on 1 June and Hungary on 10 June. "I am looking forward to the time in Turkey because there I will be able to work with the players every day, and most of the time it will be two times a day".More news: Light rains lash several parts of Pb, Hry