The All Stars were voted for by supporters and marquee signing Del Piero was among the top picks, along with Japan international Shinji Ono.
However, neither will be available for the sell-out encounter at the ANZ Stadium.
Del Piero is still in Italy and will only travel for Sydney FC's pre-season friendly with Sagan Tosu on July 24, while Ono is not being released by Western Sydney Wanderers as the club are unwilling to risk their star man at such an early stage of his preparations for the new campaign.
"They won't be available," said Postecoglou.
"The panel was selected by the fans and Del Piero was the most prominent of all so it is disappointing he won't be part of it, but we kind of knew with the timing of the games it would be difficult to get everyone involved who the fans wanted.
"But we are still pretty confident the team we put out will be a good side."
United are due to fly to Sydney for the second leg of their five-match summer tour on July 14, the day after David Moyes' first match as Red Devils boss in Bangkok.