PHILADELPHIA, PA - OCTOBER 20: Ben Simmons #25 of the Philadelphia 76ers looks on against the Boston Celtics at the Wells Fargo Center on October 20, 2017 in Philadelphia, Pennsylvania. NOTE TO USER: User expressly acknowledges and agrees that, by downloading and or using this photograph, User is consenting to the terms and conditions of the Getty Images License Agreement. (Photo by Mitchell Leff/Getty Images)

Aussie Philadelphia 76ers star Ben Simmons is set be ruled out indefinitely with a back injury, Sixers coach Brett Brown confirmed on Tuesday.

The two-time All-Star hurt his back last week during a Sixers practice session.

He missed Philadelphia’s win against the Brooklyn Nets on Friday (all times AEST) and only played five minutes in Sunday’s loss to the league-leading Milwaukee Bucks.

Simmons had reportedly undergone an initial evaluation on Monday in Philadelphia.

ESPN’s Adrian Wojnarowski reported that Simmons will be sidelined for an “undetermined period of time” adding that the injury will have “longer-term implications than simply the 76ers’ upcoming road trip”.

“The 76ers expect there to be an answer within the next 24 hours as to what the course of action will be as far as remedying the back injury.” Wojnarowski said.

With Philadelphia heading into the back half of a season where they are battling for a top four spot in the Eastern Conference, Brett Brown was cautious when talking about Simmons.

“It’s really, like, how long is a piece of string? Who knows?” Brown said to reporters.

If ruled out for the remainder of the season, Simmons’ involvement with the Australian Boomers at the Tokyo Olympics in July could be in question. Simmons missed the Basketball World Cup in September last year where the Boomers finished 4th, being beaten in the semi final by eventual winners Spain.