Postecoglou has won just two of his 12 matches in charge of the national team, prompting a worrying slide down the FIFA rankings.
With a major tournament on home soil now less than a month away, some observers have questioned Postecoglou's experimentation and called for a more pragmatic approach.
But Kewell, who hung up his boots at the end of last season, feels the former Brisbane Roar and Melbourne Victory boss knows what he's doing.
"I think a lot of people should just be quiet and let Ange get on with his job," he was quoted as saying by News Corp.
"He's a tactical man and the one thing with football is how many people have opinions about it.
"They should just let him get on with his job and let him concentrate on the Australian team in the Asian Cup and hopefully he can do us proud and bring home the trophy.
"Everyone is allowed an opinion, but I always let my football do the talking and it's the same as Ange and the Socceroos.
"I'm sure they'll let people know they really mean business.
"You should leave it up to the people who actually know the game."
With a major tournament on home soil now less than a month away, some observers have questioned Postecoglou's experimentation and called for a more pragmatic approach.
But Kewell, who hung up his boots at the end of last season, feels the former Brisbane Roar and Melbourne Victory boss knows what he's doing.
"I think a lot of people should just be quiet and let Ange get on with his job," he was quoted as saying by News Corp.
"He's a tactical man and the one thing with football is how many people have opinions about it.
"They should just let him get on with his job and let him concentrate on the Australian team in the Asian Cup and hopefully he can do us proud and bring home the trophy.
"Everyone is allowed an opinion, but I always let my football do the talking and it's the same as Ange and the Socceroos.
"I'm sure they'll let people know they really mean business.
"You should leave it up to the people who actually know the game."