The Roar boss coached the 22-year old at U17 and U20 level for Australia and has always been a big fan of the speedy winger.
Postecoglou admits that perhaps Williams left Australia too early when he tried his luck in Denmark with Brondby in 2006 but insists that there is much more to come from the highly talented Brisbane-born forward.
"When David left for overseas I'm not sure it was the right time for him to go," Postecoglou told the Courier-Mail.
"He probably would have benefited from staying in the A-League a little bit longer, but it depends on the opportunities.
"I've always had high hopes for him and I still do. I think he's got a lot more ahead of him.”
But Postecoglou will be hoping Williams doesn’t show his best this weekend when the Roar take on the Fury at Suncorp Stadium in Week 15 of the A-League on Saturday – kick-off is at 8:15pm local time (9:15pm AEDT).