However, the Bhoys have now slipped up in consecutive matches. They lost to AZ Alkmaar but still secured their pathway to the next round of Europe.
Yet, it was their defeat in the Old Firm on Sunday afternoon that will come as the most dejecting blow.
It was Postecoglou’s first taste of a match against Rangers but he failed to deliver in his biggest moment yet as Celtic manager.
They lost 1-0 in what was a dismal outing for the Hoops as they succumbed to an immense wave of Gers pressure in the second half.
Late moments from Kyogo Furuhashi could have grabbed them an equaliser while Odsonne Edouard had a golden opportunity to score in the first half.
He missed from just a matter of yards out but he was keen to press the Rangers backline and had managed to get himself into scoring positions.
That was a slight positive from his game but one man who also failed to deliver at all at Ibrox on Sunday was David Turnbull.
The Scot has been nothing short of exceptional in recent weeks, scoring a hat-trick last week as they hammered St Mirren.
On this occasion, though, he put in a poor performance as he was dragged off with just over 20 minutes to go.
Turnbull was overpowered in midfield while his final ball and passing also left a lot to be desired. Considering that’s usually one of his biggest strengths, it should come as a huge concern for Postecoglou. After all, the young attacking midfielder was also hiding during the defeat to Alkmaar in midweek.
Turnbull will no doubt improve on this display in the weeks to come but it’s time for him to perform in a big game; when he’s not on song, Celtic struggle.