Postecoglou said afterwards: "Yeah we could have got something but that is kind of irrelevant to the point that we were nowhere near the levels we needed to be at today.
"In the first half we were way too passive and let Arsenal get into a rhythm. We hung in there, we hung in there the whole game really. Second half was a bit better but nowhere near the level it needed to be."
On changes at half-time, he said: "We needed something. The fact we went out in the first half in such a big game and were so passive, it was unacceptable. And we paid a price for it.
"When we play well and take the game to teams then we are at our best and we didn't do that today."
On Richarlison's return, Postecoglou added: "Good to get him back. He has pretty much missed the whole season. But injuries are irrelevant if we are not going out there with the right sort of mindset in the team, and we didn't today."