Spurs had shown plenty of improvement during Postecoglou's debut season until a run of three straight losses checked their progress.
Defeat at Chelsea on Thursday night highlighted some of the deficiencies in the current Tottenham squad, which has largely been put together by pragmatic managers in Jose Mourinho, Nuno Espirito Santo and Antonio Conte, but are now being asked to play a more progressive style of football.
Postecoglou has already made a number of bold adjustments, with veterans Hugo Lloris and Eric Dier discarded in January, while Guglielmo Vicario, Micky van de Ven, James Maddison and Brennan Johnson have been signed this season.
However, further changes will occur this summer, with Tottenham set to listen to offers for Pierre-Emile Hojbjerg, Giovani Lo Celso and Bryan Gil, the PA news agency understands.
"We need change. Change has to happen," Postecoglou reflected.
"You can't want to alter your course and quite dramatically for this club because we went down a certain direction. And now we're pivoting to a whole different direction, expect the same people are going to be on that. It's just not going to happen.
"It's not for the want of trying. It's just that we're going to play a certain way, we're going to train a certain way, we're going to have a certain mindset and that's not for everyone.
"And the same way, whether it's Liverpool or Arsenal, if you look at the beginning of their journeys, by the time they win the competition or have success, the team's almost unrecognisable.
"We've already made tough decisions, we let some experienced players leave.
"Some of those decisions aren't that tough because whether it's a player or a staff member, they realise it themselves and say 'you know what, I've got a better path somewhere else and you've got a better path here'.
"I've got to change this squad, I have to because I've got to build a squad I think can play our football. For that to happen, there has to be exits."
Postecoglou has faced criticism for his reluctance to change for certain opposition, but vowed to stay true to his principles of front-foot football, which includes a high line and a commitment to playing out from the back.
"I believe what I believe down to my core. I'll stand on the highest ground, die a noble death, believing in what I believe," he added.
"Like anything in life, once you feel like something is working for you, something you believe in brings success - and it does - you take that forward.
"Maybe the Premier League is a step too far, mate. Who knows? Maybe, maybe - but I will still be on that hill, wounded, my dying breath saying 'I believe, I do believe'.
"But I do, I really do mate. And as long as I'm here that's what you will see. You will see this team playing the way I want it to."
14°C