The Brumbies have confirmed first five Christian Lealiifano will miss the rest of the Super Rugby season after breaking his ankle in Saturday's win over NSW Waratahs.
The 25-year-old will undergo surgery and will be out of action for six months.
The Brumbies coach Jake White concedes it's a significant blow to their title hopes.
Lealiifano had also been pressing his claim for a Wallabies jersey after a breakout season.
Saturday's win in Canberra left the Brumbies with a nine-point buffer at the top of the Australian conference ahead of the bye this weekend.