"The healing that needs to be done is pretty significant," Mitchell told AAP on Tuesday after leaving New Zealand with his leg in a moonboot.
"I'm not too sure if I'll be lacing them up in the next 12 months or so."
The news is a huge personal blow for the 28-year-old who has played very little rugby since suffering another nasty ankle injury during the Waratahs' Super 15 clash with the Reds in April of last year. Chasing a kick downfield, Mitchell was impeded by fellow Wallaby Scott Higginbotham, with the end result being a dislocated ankle and a fractured fibula 16 months ago.
Mitchell was almost immediately ruled out of September's World Cup after being told he would require at least six months on the sidelines but he somehow found a way back to fitness earlier than expected. Hard work and healing bones led to him making the Wallaby party for the global gathering in New Zealand and when he scored a brace of tries against Russia in the final pool game, things were looking rosy once again. Fate drew Mitchell yet another cruel hand later on in that game, though, as he tore a hamstring in Nelson to limp out of the tournament before the knockout stages.
Further ankle problems then prevented him from featuring in the first four months of the 2012 Super Rugby campaign as the remnants of his 2011 setback refused to fully disappear. Mitchell missed 13 fixtures last term before finally returning to the Waratahs side for the clash with the Cheetahs in Bloemfontein in the last week in May.
His call up for Australia's Rugby Championship squad was a huge achievement given his injury woes and his chances of becoming a Wallaby regular again looked good after he was named in the starting XV for the second of the back-to-back games against the Kiwis.
That was Mitchell's first Test start in 11 months and it now looks like he could face an even longer wait to make his next one.
His absence is bad news for under-fire Wallaby boss Robbie Deans who is already without fellow wingers Joe Tomane and Cooper Vuna, while utility back James O'Connor hasn't featured since the autumn tour.
Deans could now promote Dom Shipperley of the Reds or Nick Cummins of the Force for the September 8 clash with South Africa, although O'Connor remains hopeful that he may finally be fit enough to feature in Perth.