The Socceroos will face China next week in the quarter-finals after losing 0-1 to South Korea.
"It is what it is," Postecoglou told media.
"Whatever challenges we have got ahead of us, if we perform like that … we will be hard to beat."
The coach acknowledged that a defensive error was to blame for the defeat.
"We obviously conceded a pretty poor goal from us, we switched off," Postecoglou said.
"From our perspective it was very disappointing."
The coach rested Tim Cahill, Robbie Kruse and Mathew Leckie, only using them late in the second half.
Team captain Mile Jedinak was not used and given more time to get over an ankle injury that he picked up in an earlier game.
"We put out a team that we thought could win the game … we had some fresh players and we thought they would give us the best chance of victory." he said.