Australia's spluttering World Cup qualification campaign has heaped pressure on the Socceroos boss with reports the mounting criticism has Postecoglou looking to stand down from his post after the upcoming play-off matches with Honduras.
Kewell, manager of English League Two side Crawley Town, has called out Postecoglou's detractors and insisted the much maligned national team coach should be given the full support of Australia's football fraternity.
"A lot of it is just speculation. Stuff circulated by the media. It's easy to sit behind a desk or read off a bit of paper and say someone is not good enough," Kewell said,
"It amazes me because there are a lot of people who constantly say these sort of things but don't make themselves accountable and step out into the firing lines.
"Everyone has an opinion in football but you need to trust and back you manager. He has been hired to lead and you need to support his vision.
"You're not going to agree with everyone but as long as the players take on the information and take that out onto the pitch on match day that is the most important thing."
Reports of Postecoglou's potential early exit has prompted fans to discuss what mark former Melbourne Victory and Brisbane Roar coach has had on Australian Football.
But Kewell believes any such discussions about Postecoglou are premature.
"As far as legacy it is too soon to start talking about that," Kewell said.
"Yes he won the Asian Cup, Australia's first major international trophy, but I think he has much more to do until you can start looking back on his influence on the game.
"I don't think you can just say cause it is a legacy now cause he took over four years ago. Can you create a legacy in that time?
"For me a legacy is what Sir Alex Ferguson has done at Manchester United, so no four years is not enough."
