The 23-year-old was ruled out of the one-day series last week against England with lower back soreness and scans have revealed a more serious injury.
"Mitch Starc had scans following his arrival back to Australia and they have shown an early stage low back stress fracture. His management plan will be determined in the coming days but he is expected to be unavailable for a prolonged period," Cricket Australia's (CA) chief medical officer Justin Paoloni said Monday.
However, the good news for Australian cricket fans was that right-arm pacer Jackson Bird has shown recovery from his back injury and will be available for the Ashes, which will be held from November 21 to January 7 in Australia.
"Jackson returned home from the tour of England with some lower back soreness. We have ruled out a major injury but will review scans in two weeks to confirm this. We expect that he will be available during The Ashes," said Paolini.
Commenting on the injuries, CA general manager for team performance Pat Howard said: "While it's a disappointing result for Mitch, we know he will bounce back. We are aware that until pacers get into their mid-20s they are more susceptible to injury. We're doing our best to minimise long term injuries but are also realistic that such injuries are possible."
"We have a greater reliance on younger fast bowlers at international level than ever before and we're getting the most we can out of these talented young players, who are all coming through into international cricket together."