Despite Mooy playing a leading role in Australia's 1-1 first leg draw in Malaysia on Thursday, the Huddersfield midfielder looks set to start from the bench, according News Corp reports.
It will mean Celtic midfielder Tom Rogic will start, with Mathew Leckie moving from his central role - where he starred in the first leg - to the right wing, with Josh Risdon missing out.
Meanwhile, Socceroos icon Tim Cahill is expected to start and lead the attack against Syria.
Postecoglou's controversial gamble will come under the microscope should Australia fail to overcome Syria tonight.
The Socceroos need either a win or a 0-0 draw to advance to the final play-off in November, where they will meet the fourth-placed North and Central American side, likely to be either Panama or Honduras.