It was revealed on Thursday a trial could commence soon where a blue card would be shown for dissent and professional fouls where the offenders would be sin binned for 10 minutes.
FIFA has since clarified the trial will not occur in elite football, while football's lawmakers, the International Football Association Board, will not publish plans for the sin bin trial until next month.
However, Postecoglou was unequivocal in his belief it would be a wrong move.
"One team being down to 10 men for 10 minutes, you know what it's going to do to our game? It's going to destroy it, mate," Postecoglou insisted.
"You're going to have one team just sitting there trying to waste time for 10 minutes waiting for a guy to come on.
"Every other sport is trying to declutter. All we're trying to do is go the other way for some bizarre reason."
Postecoglou remains unconvinced but added his "biggest issue" in football is VAR.
"I don't think people will be surprised by my thoughts on it. I struggle to understand this urgency all of a sudden to bring in new things," he said.
"I don't know if there's that much wrong with the game as I see it. My biggest issue with the game right now is that VAR has changed the experience, whether you're a player, a manager or a supporter or whatever you are I think it's changed the experience of football.
"I assume that's a means to an end, that the introduction of technology is going to get us to a better place. I remain to be convinced about that.
"Beyond that, I don't know why a different colour card is going to make any difference. I struggle with this whole taking from other sports."
Liverpool boss Juergen Klopp believes the proposal does not "sound like a fantastic idea."
Klopp believes a blue card could present "more opportunities to fail".
"I think we should keep it as simple as somehow possible, for the referees as well," he said.
"It's a difficult job, often quite emotional when we speak about it more so because it's after the game, and I think the introduction of a blue card would just give more opportunities to fail as well.
"Because the discussion will be, 'it was a blue card, should it have been a yellow card?'.
"Now it's 10 minutes off - in the good old times it would have been a red card or only a yellow'.
"It doesn't sound like a fantastic idea in the first moment but actually I can't remember the last fantastic idea which came from these guys, if they ever had one. I am 56 and - pah, never!"
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