Lambie, who has starred at both five-eighth and fullback for the Sharks, was one of three key players left at home as the Durban-based outfit flew out to Brisbane for Saturday night's clash.
The Sharks failed in their bid to have Test inside centre Frans Steyn, a late-season recruit, made eligible for the finals while rising lock Piet-Steph du Toit sustained an ankle injury in their last-round 34-15 win over the Cheetahs.
Lambie aggravated an ankle problem against the Cheetahs in Durban as the home side fought back from 15-6 down at halftime to score four second-half tries and knock the Brumbies out of the play-offs.
While the Reds sweated on the availability of cited playmaker Quade Cooper, who was the face a judicial hearing on Monday afternoon, the loss of Lambie was a significant help to the defending champions' cause.
Former French five-eighth Frederic Michalak has shown solid form at No.10 but the Sharks have looked at their best when Lambie was pulling the strings.
The powerful Louis Ludik looms as Lambie's likely replacement at fullback.
Steyn was ruled ineligible for post-season matches because he was signed by the Sharks after the April 1 transfer deadline, forcing a new centre combination.
Lock Steven Sykes and prop Tendai Mtawarira have both been passed fit after being late withdrawals against the Cheetahs while flanker Jean Deysal was left out of the 24-man squad.
Key to their revival in the comeback win over the Cheetahs was the injection of Springbok forwards Bismarck du Plessis and Willem Alberts off the bench.
The Sharks travel to Brisbane as underdogs but coach John Plumtree is adamant they can continue a late-season surge which has seen them win six of the past seven matches, all in South Africa.
"Everyone says you can't win on the road, but we're going to prove everyone wrong," Plumtree said in Durban.
The Natal franchise have done it before in a Brisbane play-off, upsetting the minor premier Reds in 1996 at Ballymore to make the inaugural Super 12 final.
Sharks squad: Meyer Bosman, Paul Jordaan, Louis Ludik, Charl McLeod, Frederic Michalak, Lwazi Mvovo, Odwa Ndungane, JP Pietersen, Cobus Reinach, Riaan Viljoen, Tim Whitehead; Willem Alberts, Jacques Botes, Anton Bresler, Craig Burden, Marcell Coetzee, Keegan Daniel (capt), Bismarck du Plessis, Jannie du Plessis, Wiehahn Herbst, Ryan Kankowski, Jandre Marais, Tendai Mtawarira, Steven Sykes.