Coach JP Pietersen confirmed on Thursday that the 28-year-old will face an extended layoff after sustaining the injury against the Lions at Ellis Park last week, adding that the full extent of the injury is still being assessed.
"Fassi saw a surgeon this week and we're just waiting to hear if it's surgery or rehab," he said.
"I don't want to speculate, but it could be around six weeks if it's rehab, or longer if he needs an operation. We'll have more clarity soon."
It's the latest injury blow for Fassi, who suffered ankle injury in the Springboks' 43-10 victory over the All Blacks in Wellington in September last year and underwent surgery, missing the remainder of the 2025 Rugby Championship calendar and the end-of-year tour before making his long-awaited return for the Sharks in mid-December.
In Fassi's absence, Pietersen has shifted winger Jaco Williams to 15 for Saturday's match against the Bulls at Loftus Versfeld.
The coach is confident the 20-year-old former Junior Bok star - who earned an invite to the Springbok's first alignment camp of the year earlier this week - is ready for the challenge, having spent much of his career in that position.
"Jaco is a fullback. That's his position, so we've got no problem with him moving there."
Pietersen recalled Springbok prop Ox Nche and centre Andre Esterhuizen, but a number of Shark Boks - including Grant Williams, Ethan Hooker, and Siya Kolisi - are still absent due to their rest protocols.
"Andre was always going to play this weekend. He's our captain. Ox was also always going to be back. It's part of our rotation," Pietersen said.
Siya Kolisi is confident the next generation of Springboks will be ready to step up when needed, and says the back-to-back World Cup champions are "never satisfied" despite their recent success.
Springbok assistant coach Felix Jones claims that contestable kicks do not lead to more scrums as many detractors of the tactic suggest.
Springbok coach Rassie Erasmus is a satisfied man after the team's opening alignment camp of the year, which concluded on Thursday evening alongside the SA Rugby Awards in Cape Town.
Springbok hooker Malcolm Marx capped off a fantastic season when he won the SA Rugby Men's Player of the Year Award, while Nadine Roos did the double by being named SA Rugby Women's Player of the Year for a second consecutive year.
