Postecoglou was happy with the defender in defeat at home to Arsenal on Sunday.
"He was outstanding," Postecoglou said of Romero.
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"He's a World Cup winner and I've just got to get some of what's in him into some of the others."
He also said: "For us it's about, when I was focused on the details of, not just set-pieces but a lot of moments in games where we don't sense that you give good opposition the time and the space to do things then they're going to hurt you.
"I think they maybe had four attempts on goal in the first half and they conceded three. I don't think it's about one part of it, I think it's a bigger, broader picture than that, but our defensive set-pieces for those two were very poor. But there's a lot more than that to fix."