David Pocock was ruled out for nine months following knee surgery in a shattering blow for the key flanker and Australian captain.
It is a major setback for Pocock who will miss Australia's Test series against the British and Irish Lions as well as the Wallabies' internationals in the Rugby Championship against New Zealand, South Africa and Argentina.
The star forward suffered the knee injury playing for the Brumbies in their 35-6 Super Rugby win over the Waratahs in Canberra last weekend.
He fell awkwardly in a tackle by fellow Wallaby Adam Ashley-Cooper early in the first half of the game and was replaced by former Wallaby flanker George Smith.
Wallabies coach Robbie Deans will now have to find a flanker replacement with the early contenders likely to be the Waratahs' Michael Hooper or Liam Gill of the Reds.
The latest setback comes after Pocock had surgery on his other knee last year, which ruled him out for three months.
"I'm pleased to have the surgery out of the way. Now I need to rest up and get myself ready to start the rehabilitation process," Pocock said in a statement.
"The key thing is that I'm patient and don't try to rush back. If that means season over, in terms of playing any more rugby this year, then so be it. Once I get into the rehabilitation, we'll see how it goes."