Stormers will be without prop JC Janse van Rensburg for 12 weeks because of a knee problem.
Van Rensburg suffered the injury in Stormers' 17-17 draw with Sunwolves last weekend.
Coach Robbie Fleck said on Sunday that the prognosis was not good for Van Rensburg.
And he now faces three months on the sidelines in a substantial blow to a Stormers team that has the third-best record in Super Rugby having lost just three of their opening 10 games.
Meanwhile, centre Juan de Jongh will miss Saturday's clash with Bulls due to a hamstring strain.
A post on Stormers official Twitter account read: "JC Janse van Rensburg out for 12 weeks with a MCL knee injury. Juan de Jongh has Grade 1 hamstring strain, misses Bulls game."