Cricket Australia had previously announced that a back injury had ruled the Australia captain out of their upcoming tour of India starting on October 13, and Clarke himself has now admitted that he could miss the first Test of the Ashes series.
Asked whether he could miss the first Test at the Gabba, he said: "I'll say definitely not (a chance of missing), but at this stage, we can't predict how long it's going to take to get right.
"I'm trying not to look at that. It's about being nice and positive."
Team physiotherapist Alex Kountouris revealed that Clarke's chronic lower back injury flared up during the recent one-day series in England and had been slow to improve.
"Michael will require a more extensive rehabilitation period before returning to competitive cricket," Kountouris said.