Kolisi tore ankle ligaments iin Saturday’s 28-3 win over the Cheetahs on Saturday and will be out of action for six weeks.
"It’s the least problematic ligament, the anterior one, so no surgery will be required, but he will out for six weeks," Stormers coach Allister Coetzee said on Monday.
"The loose forward (injury) bogey doesn’t want to go away," added the coach,
"But, I guess, we should be thankful that it’s not as serious as the other guys (Eben Etzebeth, Peter Grant, Michael Rhodes) in terms of their ankle injuries.
"It happened in the first half (against the Cheetahs), but Siya didn’t even feel it.
"He felt it on Sunday only… so, yes, it seems like we have a curse at loose forward!"
Kolisi was due to return the week South Africa opens
its Rugby Championship campaign at home to Argentina on August 17.
The Stormers said Kolisi did not require surgery.
The 22-year-old loose forward made his Springboks debut against Scotland last month, winning the man of the match award after coming on as an injury replacement in the opening minutes.
The Stormers have two regular-season games left in Super Rugby and only an outside chance of making the playoffs.
The Stormers travel to Port Elizabeth to play the Kings on Saturday (kick-off at 17:05), before concluding their campaign against the Bulls at Newlands (Saturday, July 13 at 19:15).