Adelaide United youngster McGree capped his meteoric rise from relative obscurity on Tuesday by sealing a spot in Australia's final 23-man squad to play Iraq in neutral Tehran on Thursday, March 23 and the UAE in Sydney five days later.
McGree's selection comes after just 12 A-League and two Asian Champions League appearances since breaking into the struggling Reds first team to fill various midfield positions, largely an unfamiliar holding role.
Postecoglou was unconcerned McGree's experience amounted to less than a single professional season, labelling him an exciting future prospect.
"We haven't had in the A-League too many teenagers even getting a run these days ... I've never felt like I've needed to sit on these things," Postecoglou said.
"I've always felt if there's somebody there I want to have a look at I'll bring them in.
"I'm not going to wait because in the past we've just waited too long, particularly with some of our younger players.
"If he does well at training he could play. Obviously I want to bring him in to have a look at him in the environment.
"But we are there to play well and win some games and if he looks like he's adjusted well in training and looks bright then we won't be afraid to throw him in."
Should McGree get any minutes he'll be nine months older than the youngest - and perhaps greatest Socceroos player - Harry Kewell, who debuted at 17 years and seven months.
Uncapped Sydney FC duo Rhyan Grant and Danny Vukovic also survived the cut as did Perth's Rhys Williams, Melbourne Victory's James Troisi and Denmark-based talent Mustafa Amini.
The seven players axed from the extended 30-man squad were Nathan Burns, Ryan Edwards, Alex Gersbach, Craig Goodwin, Chris Ikonomidis, Awer Mabil and Matt McKay, though Postecoglou hinted at a Goodwin recall in the case of front-third injuries.
The coach acknowledged he took a punt on central defender Trent Sainsbury, who is yet to graduate from Internazionale's training pitch, and predicted he and Bournemouth bench regular Brad Smith would likely get through one game each.
He was also confident veteran Tim Cahill would have an impact despite his limited game time for Melbourne City recently.
Cahill is one of only two out-and-out strikers alongside likely starter Tomi Juric.
The Socceroos (nine points) enter the crucial double-header in a precarious position, third in Asia's Group B behind Saudi Arabia and Japan (both 10 points) and outside the top-two automatic qualification zone for Russia 2018.
There's little margin for error thanks to November's lacklustre 2-2 draw with stragglers Thailand - an Australia win in Bangkok would have lifted them into first and provided an advantage for the second half of the final phase.
UAE are fourth but equal on nine points with Australia while Iraq are fifth on three points.
Australia: Mustafa Amini, Aziz Behich, Tim Cahill, Milos Degenek, Rhyan Grant, Jackson Irvine, Mile Jedinak, James Jeggo, Tomi Juric, Robbie Kruse, Mitch Langerak, Mathew Leckie, Massimo Luongo, Riley McGree, Mark Milligan, Aaron Mooy, Mathew Ryan, Trent Sainsbury, Brad Smith, James Troisi, Danny Vukovic, Rhys Williams, Bailey Wright.