The 27-year-old was a regular starter for his country before be fractured his ankle and broke a bone in his leg in a horrific clash with Scott Higginbotham when the Waratahs took on the Reds in April of last year.
Mitchell missed most of the domestic season as well as Australia's title-winning Tri Nations campaign and was initially ruled out of the entire World Cup. He recovered quicker than expected, however, and scored three tries in as many pool matches before fate struck him a second cruel blow.
Another ankle knock brought his tournament to a premature end after the win over Russia, with a bone spur then eventually being removed in early December.
A torn tendon has since further delayed his comeback and he is now resigned to missing the opening game of the new Super 15 season.
"It would definitely be the toughest thing I've had to go through with my career," said Mitchell, who has 44 Test caps to his name but has spent much of the past nine months training on his own.
"I guess I know my body pretty well and I'm at the point now where I'm resigned to the fact I won't be playing in that first (Super 15) game.
"When you feel like you're going to be back at a certain point and you have another setback and you feel like you're going to be back from that and then you have another, it tests you and throws you around a bit.
"You get anxious about when you are going to be back and if you are going to be back. When you get back, are you going to be back to where you were prior to that? Also, when you get back, how are selections going to go, so that comes into it as well.
"It is tough, but I'm certainly not the first guy to be tested in this way."