Broad was struck on the right foot by Australian fast bowler Mitchell Johnson on the third day of the third Test, as the match and series looked to be sliding away from the beleaguered English on another scorching Perth day.
He did not take to the field during Australia's second innings, and headed instead to a local hospital for scans on his foot after briefly trying to test the injury by bowling in the nets.
The 27-year-old said he would not bowl during the Australian second innings, but was willing to bat in England's second innings if needed.
He said he hoped to be fit for the fourth and fifth Tests in Melbourne and Sydney.
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"Something showed up on the X-ray, but it was a bit inconclusive, so I had to have an MRI," he said.
"We are just waiting on those results.
Broad was hopeful that the extended break before the fourth Test, which starts on December 26 at the Melbourne Cricket Ground, would work in his favour.
Broad is England's leading wicket-taker in the series, his haul of 14 double that of Jimmy Anderson.
His injury capped a horror day for the English, with Australia leading by 369 runs with seven wickets in hand and two days to play.
"It was a bad one, there is no hiding away from that," Broad said.
Australia will claim the Ashes for the first time since 2007 with victory in Perth, but Broad said the cause was not yet lost.
"There is a lot of belief in that changing room and there is a lot of guys with great Test records in that changing room that haven't delivered so far but are due to.