Lambie was laid flat in a high tackle by Ireland flanker CJ Sander in the first test between the two countries at Newlands in Cape Town on June 11 and has not been able to return to training since.
The 25-year-old had been expected back for the closing stages of the Super Rugby competition but the Sharks' medical staff said symptoms of the concussion had not yet cleared.
"He is being monitored on a daily basis and our medical staff are doing everything they can to get him back on the field," a statement from Sharks doctor Alan Kourie said.
"But there is uncertainty about when he will play again."
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Lambie had been the first choice for the Springboks this year at flyhalf after injury to Handre Pollard, who has had knee and shoulder surgery.
South Africa this week called up 19 players for a camp in Stellenbosch and will on Saturday name their squad for this year’s Rugby Championship against Argentina, Australia and New Zealand.
(Reporting by Mark Gleeson; Editing by Alison Williams)
