Cahill was a shock omission from November’s squad that drew 2-2 with Thailand due to fitness issues surrounding an Achilles issue, and Postecoglou is concerned about the veteran striker’s lack of game time for Melbourne City.
The 37-year-old has been named in an extended 30-man squad but has played just 49 minutes in the last month and had no role in last weekend’s 3-2 win over Central Coast.
He was suspended for one game during that period.
City don’t play this weekend as part of the A-League’s split round, leaving Cahill with no opportunity to impress the national team coach before he whittles his squad down early next week for the March 23 game in Tehran and March 28 fixture in Sydney.
“It is a concern,” Postecoglou said.
“You want all your players playing regularly. I thought Timmy would have had more game time over the last couple of weeks.
“We still have two weekends to go (before the games), we’re already monitoring it closely again.
“Timmy’s one who has done well in the past even if he hasn’t had game time, he’s come in and still had an impact for us, so we’ve factored that in.”
Cahill is one of four players on notice along with FC Tokyo attacker Nathan Burns, Internazionale centre-back Trent Sainsbury and Bournemouth left-back Brad Smith, none of whom started for their clubs last weekend.
“We’ll see how they go over the weekend, but it’s unlikely all four will make it if they don’t play significant minutes over the weekend,” Postecoglou said.
Eighteen-year-old Adelaide United midfielder Riley McGree is the shock bolter, the sprightly teenager selected on the back of just 710 minutes in the A-League.
Veteran goalkeeper Danny Vukovic and right-back Rhyan Grant have also been rewarded for their career-best form with Sydney FC as Postecoglou upheld his pledge to include more A-League players.
The trio are part of seven uncapped players alongside Awer Mabil, Jimmy Jeggo, Ryan Edwards and Mustafa Amini.
Tim Cahill and Tomi Juric are back while Asian Cup hero James Troisi returns for the first time since 2015 and Perth defender Rhys Williams and Brisbane captain Matt McKay have been recalled.
Playmaking wizard Tom Rogic is absent through injury while China-based central defender Matthew Spiranovic is the biggest casualty from the last squad.
Jamie Maclaren, Apostolos Giannou, Josh Risdon and the injured Adam Federici also miss out, though Postecoglou kept faith with attackers Robbie Kruse and Nathan Burns despite their seasons in Asia only just starting.
Australia sit third in their qualifying group and have five matches to secure a top-two finish and automatic qualification for Russia 2018.
The final squad will be trimmed down to 23 players.
Australian squad: Mustafa Amini, Aziz Behich, Nathan Burns, Tim Cahill, Milos Degenek, Ryan Edwards, Alex Gersbach, Craig Goodwin, Rhyan Grant, Chris Ikonomidis, Jackson Irvine, Mile Jedinak, James Jeggo, Tomi Juric, Robbie Kruse, Mitch Langerak, Mathew Leckie, Massimo Luongo, Awer Mabil, Riley McGree, Matt McKay, Mark Milligan, Aaron Mooy, Mathew Ryan, Trent Sainsbury, Brad Smith, James Troisi, Danny Vukovic, Rhys Williams, Bailey Wright.
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