The Stormers have announced that injured pair Duane Vermeulen and Rynhardt Elstadt are flying home, with Gerbrandt Grobler joining the squad.
Both Elstadt and Vermeulen limped off in the final ten minutes of the Stormers' 15-21 Super Rugby defeat to the Waratahs in Sydney on Saturday and the duo face up to two months on the sidelines.
Grobler left for Australia on Sunday, with Elstadt and Vermeulen going to South Africa on Monday.
"Both Rynhardt and Duane will be out for six to eight weeks - both suffered Medial Collateral Ligament (MCL) injuries," confirmed Stormers coach Allister Coetzee to the franchise's website.
"It's a massive blow for us at this stage of the season."
In the absence of Elstadt and Vermeulen, Don Armand and Nizaam Carr could move into the starting line-up against the Rebels next Friday, with Grobler - fresh off a strong Vodacom Cup campaign for WP - set to provide cover from the bench.
"As we saw in the final ten minutes against the Waratahs - losing Rynhardt and Duane had a big effect on the final result," added Coetzee.
"We really will miss their influence - on defence and with ball in hand - but I have full confidence in Don and Nizaam to step up into starting roles should they be required to do so."
Second-row forward Grobler, 21, has made one appearance for the Stormers this season, coming off the bench in his team's 22-15 win over the Sharks a month ago.