George Smith is poised to become the Brumbies' third captain in as many weeks with Stirling Mortlock in serious doubt to play the Sharks on Sunday (AEDT).

Mortlock is rated a 50-50 chance at best of lining up at Durban's Absa Stadium after his recovery from a knee injury has stalled in the past two days.

The Wallabies skipper was cleared of any structural damage in the joint after an ugly collision in the 25-17 loss to the Lions but the cork is preventing him from striding out in training.

Brumbies coach Andy Friend is prepared to give the inspirational centre until match day to prove his fitness and has delayed naming his team until Thursday.

"Given he's such a quality footballer we'll give him up until the last hour," Friend said from Durban.

Mortlock, himself, admitted concerns over the slow recovery.

"It improved quite well over the two days after the game but since then it's stagnated," said the 110-match Super rugby veteran.

"It needs to improve in the next day and a half or I'm at long odds I would say."

Smith is the obvious choice to replace Mortlock as captain after they were the only two considered for the role when skipper Stephen Hoiles sustained a knee injury against NSW.

Adam Ashley-Cooper looms as the new No.13 with Clyde Rathbone in line to be promoted to start on the wing against the team he debuted in as a 20-year-old in 2002.

Friend also delayed naming his team to assess lineout expert Mitchell Chapman, who is battling to return from shoulder surgery.

While Mortlock and Chapman require fitness tests, Ashley-Cooper (ribs), Mark Gerrard (elbow) and Julian Salvi (corked thigh) have all been cleared to play.

The third-placed Sharks will start a golden run home to the finals against the Brumbies with this being the first of seven matches in South Africa, including six in Durban.

Brought to you by AAP © 2024 AAP