Hanro Liebenberg has been ruled out for six to eight weeks after suffering a knee injury against the Sharks.
The Bulls No 8 damaged a meniscus ligament and will undergo surgery on Wednesday.
In some good news for the Bulls, wing Travis Ismaiel (back niggle) has started selected team sessions, while flank Nic de Jager has recovered fully from a rib injury.
The Bulls, who thrashed the Sharks 40-10 at Kings Park, will host the Rebels on Saturday.
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