Postecoglou said: "It's a tough one, a bit raw at the moment. Credit to Newcastle, I thought they were really good today. We never really got to grips with the game and weren't able to execute any of the stuff we usually do. Credit to them but just have to take them.
"I thought even before (the quick goals) that we lacked a little bit of conviction today throughout a lot of things. They can hurt you particularly in transition and they did that today."
On needing to bounce back, he added: "You've got no choice. You can't sit there and feel sorry for yourself, it's not the nature of what we do. There's some lessons in there, we've got to take them and look ahead."
On Micky van de Ven's mistakes, the manager said: "I think for all of them that's the life of an elite footballer, you're going to make mistakes. I'm sure he's going to make many more of them in his career. It's about how you react to those mistakes. I think all of us as a group have a fair bit to learn from today."