In a move that surprised many, Australia's leading all-time scorer - who would have fancied his chances of adding to his 39-goal tally from the start against meek opposition - didn't appear until the 61st minute, with the Socceroos already 5-0 up in Perth on Thursday night.
Postecoglou revealed that he left the 35-year-old frontman out in favour of a more mobile strike force - comprised of Mathew Leckie, Nathan Burns and Massimo Luongo - to tackle a side which took up permanent residence in its own 18-yard box.
Showing that reputations count for nothing and even the captaincy doesn't guarantee you a place in his team - the armband went to Mark Milligan instead - Postecoglou explained: "We wanted to get mobile and technical players out on the pitch.
"We pushed Massimo Luongo further up the pitch. He is really good in the box. I thought we had plenty of movement.
"With Cahill we just felt that we wanted to be really mobile tonight and get at them early on.
"Timmy came on and he was like a bull at the gate, ready to go. We have got him for the second game (against Tajikistan in Dushanbe) now, which is good for us."
Postecoglou gave Cahill his unreserved backing as he continues on what could be a record breaking run towards a fourth FIFA World Cup finals appearance, adding: "I am not pushing Timmy into retirement.
"He's still got lots to offer. But we will be constantly evolving the team.
"It was about giving an opportunity to some guys that need some more experience at international level.
"Tim showed when he came on he's still a handful, and he will be a great asset for us in the next game.”
Bundesliga-based Leckie, who scored his second goal for his country in the rout, thrived in Cahill's central role, even conjuring several Cahill-esque headers complete with hang-in-the-air leaps.
"Leckie is really important to us. He looked dangerous. For the things we measure as a team he's been a really important part of what we do because he works so hard in an attacking and defensive sense," said Postecoglou.
"He's always a threat. He is one of the most disciplined in his role. He knows what he needs to do in his position and he does it all the time.
"He's a striker and all strikers get frustrated when they are not scoring goals and I was pleased he got one tonight, and he could have had a couple more."
While Australia lost some of its zip and zest after surging to a 4-0 lead inside 28 minutes, Postecoglou was more than happy with what he saw in a near perfect tune-up for Tajikistan.
"It was pleasing, we got off to a really good start, which was important, and we got on top of them early," he said.
"We were pretty happy ... we hit the crossbar a couple of times and the goalkeeper pulled off a few good saves.
"For the most part we kept our discipline. It got a bit frustrating at times when the opposition sat back and there are a few stoppages which don't allow you to get into rhythm.
"Overall, I can't fault the players in terms of maintaining their focus and trying to play our style of football to the end."
