Postecoglou has confirmed Palace skipper Jedinak will pull on the armband for the game at The Den on Thursday morning (AEDT).
MORE: Cahill rested and raring to go against Ecuador | Australia v Ecuador: Mat Ryan's time to shine
The move to install Jedinak as the probable successor to Lucas Neill also casts doubt on the likelihood of Tim Cahill to start at the World Cup.
Having been overlooked for the armband, despite talking recently about his intentions to be a real leader in the team - the veteran forward could yet find himself coming off the bench if Postecoglou opts for more energy in the final third.
Neill led the team for their 1-0 friendly victory over Costa Rica - Postecoglou's first match in charge - but was omitted from the latest squad.
"It's a reward for him," the Socceroos coach said of Jedinak's appointment.
"He's playing in one of the toughest leagues in the world, captaining his club side and playing well.
"He's an outstanding candidate to lead us tomorrow night. Beyond that Mile is in the same boat as everyone else - he needs to continue to play well and continue to show form."
Barring injury or a serious loss of form, Jedinak - already one of the first names on the team sheet - now appears almost certain to start in Brazil.
After all, a season spent chasing shadows against the Premier League's best is surely ideal preparation for daunting assignments against Spain, the Netherlands and Chile.
If Postecoglou sticks with the 4-2-3-1 formation he employed in the win over Costa Rica, that leaves one more space alongside Jedinak in holding midfield.
Mark Milligan, Postecoglou's captain at Melbourne Victory before his departure to the national team, is enjoying another strong season despite his club's inconsistencies.
James Holland is a regular at Austria Wien and has only received a handful of opportunities at international level, not always with great success.
Mark Bresciano is experienced and his technical ability cannot be discounted, but declining pace and a lack of game time following his FIFA-sanctioned ban for transfer irregularities may count against him.
Of the outsiders, potential debutant Massimo Luongo will have to impress Postecoglou in this week's training camp, while Jackson Irvine, Matt McKay and Oliver Bozanic all have an outside claim, although the latter more regularly plays as an attacking midfielder.
Chris Herd, capable of playing in defence and midfield, is also a candidate, but his unavailability for Aston Villa selection due to personal reasons and subsequent withdrawal from this squad may have ended his hopes of making the 23-man World Cup squad.