A South African Rugby Union insider told AFP on Monday that the World Cup-winning skipper had seen specialists and is set to meet another on Tuesday.
"Should surgery be recommended and Siya agrees, he will almost certainly not be able to play again this year," the source said.
"The alternate is to let the injury heal and that could take about four months - meaning he would be out of action until September."
That would rule him out of Rugby Championship matches against Australia, New Zealand and Argentina. The tournament has been reduced to a single round this year because of the World Cup.
It would also prevent Kolisi from facing Argentina, Wales and New Zealand in warm-up matches before the 20-nation global showpiece kicks off in Paris on September 8th.
The Springboks begin their World Cup defence against Scotland in Marseille on September 10th and also face Ireland, Romania and Tonga in Pool B.
South African media reports said the flanker suffered a partial tear to his anterior cruciate ligaments (ACL), forcing him to retire early in the first half in Durban.
It was the last home match in Sharks' colours for Kolisi, who is scheduled to join French Top 14 club Racing 92 after the World Cup.
Fly-half Handre Pollard is the Springboks' vice-captain while centre Lukhanyo Am might also be considered as a replacement for Kolisi.
An Instagram message by Kolisi's wife on Monday asked friends and followers to pray with her family, but she did not mention the knee injury.
"Hey friends! Believing for a miracle. Would really appreciate if you could take a minute out (of) your day to pray with us," wrote Rachel Kolisi.
A new era
Kolisi became the first black captain of the Springboks - a team restricted to white players for decades - when he led the green and gold against England in 2018.
It was a new era, with coach Rassie Erasmus having replaced Allister Coetzee after two disastrous seasons for the two-time world champions.
But it began disastrously with England racing to a 21-point lead before South Africa stormed back for a memorable 42-39 triumph in Johannesburg.
The Springboks won the second Test a week later to clinch the series and the Erasmus and Kolisi partnership was off to a flying start.
As South Africa flew to Japan for the 2019 World Cup, however, not even their most partisan supporters believed they would return with the Webb Ellis Cup.
An opening-match loss to New Zealand confirmed those reservations, but it was to prove no more than a bump on the road to glory.
Convincing victories over Italy, Namibia, Canada and Japan took them to the semi-finals, where they edged Wales, before thumping England by 20 points in the final.
Kolisi and his teammates returned home to heroes' welcomes with parades through many cities attracting huge, multiracial crowds.
"This team represents all South Africans, rich and poor, young and old, black and white," said Kolisi.
More glory came in 2021 when South Africa recovered from losing the first Test against the British and Irish Lions to win the three-match series.
The only regret for Kolisi was the absence of supporters as the Tests were staged behind closed doors in Cape Town during the peak of the COVID-19 pandemic.
"We missed our supporters so much because we play for them. I know every Springbok success lifts the spirits of the people," said the captain.
