Wallaby David Pocock was ruled out for nine months following knee surgery in a shattering blow for the key flank and former Australian captain.

Pocock, 24, underwent surgery on Thursday on a torn anterior cruciate ligament in his left knee and will be sidelined for the rest of the year as he completes his rehabilitation, the Australian Rugby Union said.

"I'm pleased to have the surgery out of the way," Pocock said in a statement.

"Now I need to rest up and get myself ready to start the rehabilitation process.

"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."

It is a major setback for Pocock who will miss Australia's Test series against the British and Irish Lions and the Wallabies' internationals in the Rugby Championship against New Zealand, South Africa and Argentina.

The star forward suffered the knee injury playing for the ACT Brumbies in their 35-6 Super Rugby win over the NSW 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 flank George Smith.

Wallabies coach Robbie Deans will now have to find a flank replacement with the early contenders likely to be the Waratahs' Michael Hooper or Liam Gill of the Queensland Reds.

The latest setback comes after Pocock had surgery on his other knee last year, which ruled him out for three months.

Pocock will remain in Brisbane to recover from the surgery until next week before returning to Canberra.

He has requested privacy at this time so that he can focus on the specifics of his rehabilitation.