Goals in either half condemned Celtic to a surprise defeat in the Scottish Premiership game, a match in which they struggled to get going in.
Focus instantly was brought to bear on the fact that Postecoglou had made six changes from his previous team.
On the back of a Champions League draw against Shakhtar Donetsk, the Australian boss opted to shuffle his side against St Mirren, looking for freshness to be maintained.
He concedes thought that the team selection was most likely not the right one and as such Celtic paid the price.
Postecoglou also believes the loss at St Mirren, which he is struggling to take any positives from, reinforces the need for Celtic to stay on their toes and always hit their top level.
“Potentially, it [the performance] could be a number of reasons. From our perspective, we didn’t perform in any of the areas we like to perform in and have been performing in”, Postecoglou told BBC Sportsound.
“There isn’t anything hugely positive we can take from today, including team selection, probably wasn’t right.
“There are always reminders we still have work to do.
“Every team is a potential hurdle.”
The loss at St Mirren means that Celtic, who had gone unbeaten in the league for almost a year heading into the clash, boast just a two-point lead at the top of the Scottish Premiership over Rangers.
Postecoglou’s men next face Motherwell in the league, followed by a Champions League clash with RB Leipzig.