Clarke is due to announce his squad for the game in south-east Turkey on Wednesday and Postecoglou gave a gentle reminder that Celtic are under no obligation to release players for a match that has only recently been confirmed.
They have their own plans for the World Cup break and are taking part in the Sydney Super Cup, details of which were announced as early as March.
Although Rangers have since withdrawn, Celtic will face Sydney FC and Everton, the club’s replacement for Ibrox.
The Celtic manager was speaking after midfielder Aaron Mooy was named in Australia’s squad for the World Cup, which kicks off later this month. Defenseman Cameron Carter-Vickers will know his fate when Team USA is announced today.
Postecoglou stressed that Celtic’s return to their homeland to compete in the Sydney Cup was of the utmost importance to the Parkhead club.
Skipper Callum McGregor is a mainstay for Scotland but is currently injured and hasn’t entered Clarke’s thinking for the friendly.
However, the manager would certainly have preferred full-backs Greg Taylor and Anthony Ralston, perhaps even in-form again winger James Forrest, to be included.
“The international matches coming up in this break are somehow not mandatory for us to release players,” explained Postecoglou. “From our perspective, the trip to Australia is an important part of our club football and we want to take our strongest team there.
“We will always support the national team and Steve Clarke has always been great with us in terms of his relationship with the club. But all of our players who are not going to the World Cup will come with us.
“Obviously we are taking a team to Australia. People have bought tickets and want to see the best Celtic team possible.
“Obviously we can’t take the players who are going to the World Cup, but we have obligations to the people who have spent the money we are going there with the strongest team possible and that is what we’ll do.”