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.
Van Marwijk identified right-back as a problem area for Australia, and has brought Karacic in to assess personally, on advice from his assistants.
"But we have since seen other players, so there will be a few new names in the squad". Now he's playing every week.
"He is young and must learn a lot but he is a player who can make a difference".
"I have to make a choice [by selection deadline] about the strikers".
A former Croatian U-21 global has been named in Australia's preliminary World Cup squad despite never being to the country before.
'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, ' van Marwijk said.
"He doesn't play (at the moment), maybe five or 10 or 20 minutes".
But the coach indicated Cahill's experience of playing at the highest level in front of big crowds could be invaluable. Another three players will then suffer in Van Marwijk's final cut.
"I have had long conversations with Tim and he is a special case, he is special in everything", he said.
"He is 38 and maybe one of the best-ever players for Australia".
"He is not 19 like (Daniel) Arzani - he's twice as old - but is also a player who can make a difference".
Striker Jamie Maclaren was also recalled to the squad after some strong performances for Hibernian in the Scottish Premiership. All 26 players from Van Marwijk's first selection have been retained. "In those few weeks it must be very clear for the players what we want [at Russian Federation 2018]".
The preliminary squad will be trimmed to 26 or 27 players before a pre-tournament camp in Turkey later this month before the final 23-man roster is chosen on June 4.
The final list of 23-players is scheduled to be announced in Turkey on June 3, and the team will continue their build-up for the tournament with a friendly against Hungary in Budapest June 9.
"We have travelled extensively and watched many, many matches because we want to get as much information on as many players as possible".
Australia is drawn in Group C at the World Cup alongside France, Peru and Denmark.
Australia will also face Czech Republic on June 1 in a warm-up match for Russian Federation.