The Australian star was injured on Friday (Saturday AEST) during the final training camp scrimmage at Stockton University in New Jersey.

The Sixers said Simmons rolled an ankle, and X-rays and an MRI revealed a fractured fifth metatarsal bone.

It means Simmons is in doubt for Philadelphia's regular-season opener against Oklahoma on October 26.

The forward played at Louisiana State University last season and missed game-time early during his stay with the Tigers while waiting for an ankle injury to heal.

He also left an NBA Summer League contest in July after suffering cramps in both legs.

Simmons' injury is the latest blow to a franchise that has been beset in recent years by injuries to their players.

Joel Embiid, the No. 3 overall pick in 2014, has yet to play because of surgeries on his right foot.

Nerlens Noel missed his rookie year in 2013 with a torn ACL.

Jahlil Okafor had his rookie season cut short with season-ending knee surgery on his right knee.

It's become a bit of a theme in Philly the past few seasons, ending with last season's 10-72 mark that earned them the No. 1 pick.

Simmons became an instant fan favourite before he ever played a game and expectations were high that he was the cornerstone player that could eventually help turn the Sixers into playoff contenders.

"It means a lot, just to see people wearing my jersey around Philly, it's kind of weird,'' he said this week.

"I'm just looking forward to hopefully giving back to Philly.''

He'll have to wait.

Simmons was scheduled to start for the Sixers in their pre-season opener Tuesday against Boston.

Simmons, was Philadelphia's first No.1 pick since they drafted Allen Iverson in 1996.

He led the Sixers to their last NBA finals appearance in 2001.

Coach Brett Brown planned to use Simmons all over the court, even at point guard.

Simmons averaged 19.2 points, 11.8 rebounds and 4.8 assists, the only SEC player ever to finish in the top five in all three categories.

But the Tigers were only 19-14, failing to make the NCAA Tournament.

Simmons, who grew up in Australia before moving to Florida for three years of high school at Montverde Academy, signed a $US20 million ($A26 million) sneaker deal with Nike before the draft.