Cape Town - Stormers coach John Dobson on Tuesday confirmed that captain Siya Kolisi has suffered a Grade 3 medial collateral ligament (MCL) tear.
"Siya is unfortunately going to be out for - at best - six to eight weeks. He saw a specialist yesterday," Dobson said at the Stormers' High Performance Centre in Bellville.
Kolisi was injured in a late tackle by Hurricanes hooker Ricky Riccitelli at Newlands over the weekend.
The good news was that the Springbok captain would not require surgery.
The Stormers also lost hooker Bongi Mbonambi to a hamstring injury and Dobson confirmed that the Bok No 2 would not be available for this weekend's home game against the Bulls.
"Bongi's looking better than we thought. He won't play this week... and he's going to be out for the two to six week range," the coach added.
Scrumhalf Herschel Jantjies sat out training on Tuesday due an upper chest injury but is expected to take the field against the Bulls on Saturday.
"He's got a sterno thoracic (injury). He's sore but he'll be okay for Saturday," Dobson said.
The Stormers team will be named on Thursday, with Saturday's derby at Newlands scheduled for 17:15.