Sammie Szmodics and Liam Delap saw Ipswich 2-0 ahead at halftime before Rodrigo Bentancur struck after the break.
Postecoglou later said, "Hugely disappointing. We started slow and passively. We gave ourselves a mountain to climb. Second half we had chances but didn’t do enough to win the game.
"It's just down to me. I’m not getting consistent performances from the players. It’s something I need to address. I’m the person in charge so that’s usually the way it goes. I take responsibility.
"We can’t give ourselves those kind of mountains to climb."
On Ipswich's players going down injured regularly late in the game, he said: "It seems to be the way the Premier League is going. Clubs are taking strategic time-outs. We just have to cop it. It’s up to the officials not me."
Asked what can be fixed after the defear, Postecoglou added: "They all go away so there's nothing I can address in the next 13 days until I get them back."