The stakes could barely be higher for Australia, which needs to beat Honduras over two legs to book a place at next year's tournament in Russia.
Cahill caused consternation when he rolled his ankle in A-League action last weekend, delaying his travel.
The 37-year-old, who helped the Socceroos into the playoff with two goals against Syria last month, has yet to train fully in San Pedro Sula.
But Postecoglou believes he'll be able to call on his star forward should he need to.
"Medicos are telling me that he's going fine and he's a chance," he said.
"You're always nervous when players get injured, whether that's Timmy or anyone else. But again that's the nature of international football so you've always got contingencies.
"There's no fitness test, he'll train tomorrow (Friday AEDT) and if he's right, he's right and if he's not, he's not."
Tomi Juric is expected to start up front, with Cahill in contention as an impact substitution.
There are plenty of other decisions to make regarding his lineup for the first leg, including whether Mile Jedinak starts.
With Mark Milligan suspended, Jedinak's presence is required in midfield after a five-month absence from the Socceroos squad.
Postecoglou spoke as if Jedinak was locked in for a first start since June, a 3-2 win over Saudi Arabia in Adelaide, and only his participation in the second leg was in question.
"Mile doesn't have as many match minutes, it will be a case of seeing how he gets through the game," he said.
"He's feeling really good and he'll want to play as a big a role as he possibly can. We'll see how he gets through the game and we'll make the (second leg) decision."
Aaron Mooy, Massimo Luongo and Jackson Irvine could partner Jedinak in central midfield, with Mooy, James Troisi and Nikita Rukavytsya in the mix for attacking midfield roles.
Postecoglou suggested Tom Rogic could be saved for home soil, saying "he's one where we'll use as we have in the past, where we think it's important.".
Irvine had a stomach bug but trained two days out from the clash and is considered ready to go.
Postecoglou said injured pair Robbie Kruse (knee) and Mathew Leckie (thigh) were training alongside Milligan at Melbourne Victory this week and would join the squad in Sydney on the weekend.
Goalkeeper Mat Ryan is a certain starter.
POSTECOGLOU'S BIGGEST SELECTION ISSUES
THE BACK THREE
Trent Sainsbury is a lock, with Milos Degenek and bolter Matt Jurman battling with Bailey Wright and Ryan McGowan for two positions.
IS JEDI FIT?
If Postecoglou believes Jedinak is fit enough, there's no question: Mile starts. But who to pair him with in defensive midfield? Aaron Mooy and Massimo Luongo are the leading candidates.
THE WIDE ROLES
Aziz Behich should start on the left given his form in Turkey. In Mat Leckie's absence on the right, Josh Risdon should earn his fifth cap.
To save Tom Rogic for the second leg? Is Mooy or James Troisi the man to unlock Honduras' defence? Can Jackson Irvine play as a 10? And how best to use Nikita Rukavytsya's pace? It's here Postecoglou has the most questions - and options.