"Ultimately, we decided it was best that I use the time in September to return to Philadelphia to acquaint myself with my new teammates and prepare for the upcoming NBA season," Simmons said in a statement.
He added, however, that if Australia qualifies for the 2020 Olympics at the World Cup, he would "be honored and humbled to represent my country."
Simmons' exit leaves NBA names such as Joe Ingles, Patty Mills, Matthew Dellavedova, and Andrew Bogut on the Boomers' depth chart. Australia is in a tough group in China, alongside Canada, Lithuania, and Senegal. However, if the team fails to qualify for the Olympics at the World Cup, it can still reach Tokyo next year by winning FIBA Oceania, a region it's historically dominated.
Simmons just signed a reported five-year, $170-million contract extension with the 76ers that takes effect in the 2020-21 season.