The 27-year-old had only returned to full fitness in time for the tournament after he dislocated his ankle in April.
And he looked to be close to his best form as he crossed for a try against the USA and then for two in the win over Russia.
However it was it the process of breaking away for what would have been his hat-trick try against the Bears that Mitchell pulled up sharply, grabbing his left hamstring.
A subsequent scan has shown that Mitchell suffered a grade one tear, meaning he faces around six weeks out.
Mitchell joins No.8 Wycliff Palu is leaving the Wallabies squad, with Matt Hodgson and Lachie Turner now confirmed as their replacements.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
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The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.
