Cahill, along with a number of senior players, has seen little action at club level in recent weeks, leaving Postecoglou with a headache before he cuts the squad to 23 next week before the Iraq match in Tehran on March 23.
The Socceroos' most prolific scorer with 48 goals from 94 appearances, 37-year-old Cahill was a surprise omission from last November's qualifier in Thailand, where a tentative Australia were held to a 2-2 draw.
The disappointing result dropped the Socceroos to third in Asia's Group B behind Saudi Arabia and Japan, outside the top two spots that guarantee automatic qualification for next year's global showpiece in Russia.
Postecoglou wants to have another look at his players at club level this weekend before settling his squad but Cahill will not be able to play for his A-League side Melbourne City, who have a bye.
"It is a concern," Postecoglou told reporters in Sydney on Wednesday. "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."
Other regulars on thin ice include Trent Sainsbury, who is on loan at Inter Milan, Bournemouth left back Brad Smith and Japan-based attacking midfielder Nathan Burns.
Tall centre back Matthew Spiranovic and Tom Rogic are unavailable through injury but striker Tomi Juric, midfielder Massimo Luongo and back-up goalkeeper Mitch Langerak return after missing the Thailand match with fitness concerns.
Melbourne Victory forward James Troisi was recalled for the first time since 2015, while Perth Glory defender Rhys Williams is back in the national frame after three injury-plagued years.
Postecoglou's biggest surprise was 18-year-old Adelaide United midfielder Riley McGree, who has played only 12 games in Australia's top-flight.
"I just think he's a really exciting talent. I watched him from the first couple of games he played and he kind of makes you stand up and notice him straight away for a young guy," the coach said.
After the Iraq tie, the Socceroos head home to Sydney for a home fixture against the UAE on March 28.
(Reporting by Ian Ransom; Editing by Nick Mulvenney/John O'Brien)
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