Cape Town - Stormers coach Allister Coetzee says lock Andries Bekker couldn't cope with the tough workload a gruelling season has exerted on him.
News broke on Wednesday that Bekker suffered a back injury that is likely to keep him on the sidelines for five weeks.
"Andries had a back operation last year and we've tried to manage him this season but unfortunately he can no longer cope with the workload. Lineout work is specialist work and his injury has flared up again," said Coetzee.
"It's a big blow for us at this point as he is our lineout manager and he is a Springbok in our pack."
Fullback Joe Pietersen has still not recovered from a back injury and he too will be a non-starter for the Super Rugby derby against the Bulls at Loftus Versfeld on Saturday night.
Pietersen took a knock against the Waratahs a fortnight ago and after he missed out on the Sharks clash last week he was expected to be fit for Loftus.
"He hasn't recovered well enough to play on Saturday," said Coetzee.
De Kock Steenkamp has landed the job of lineout manager in Bekker's place and Coetzee feels he'll be up to the task.
"He has managed the lineout before and we'll see how he goes. He could be destined for that role on a permanent basis in the future."
Interim captain, midfielder Jean De Villiers will be fit, despite being ill.
"He's in bed with flu but I don't foresee a problem," said Coetzee.
"These things (injuries, sickness) one has to face in a competition as attritional as Super Rugby and we don't want it to make us negative.
Some good news on the injury front was the return of workhorse Siya Kolisi to the starting line-up. His shoulder injury has healed and he'll replace the retreaded flanker Deon Fourie in the run-on XV.
Fourie moves out to the bench as the hooker replacement at the expense of Scarra Ntubeni, who drops out of the match-day 22.
The clash at Loftus Versfeld kicks off at 19:10 on Saturday.
15 Zane Kirchner, 14 Akona Ndungane, 13 JJ Engelbrecht, 12 Wynand Olivier, 11 Bjorn Basson, 10 Morné Steyn, 9 Francois Hougaard, 8 Pierre Spies (captain), 7 CJ Stander, 6 Dewald Potgieter, 5 Juandre Kruger, 4 Flip van der Merwe, 3 Werner Kruger, 2 Chiliboy Ralepelle, 1 Dean Greyling
Substitutes: 16 Willie Wepener, 17 Rayno Gerber, 18 Wilhelm Steenkamp, 19 Arno Botha, 20 Jano Vermaak, 21 Louis Fouché, 22 Francois Venter
15. Gio Aplon, 14. Gerhard van den Heever, 13. Juan de Jongh, 12. Jean de Villiers (captain), 11. Bryan Habana, 10. Peter Grant, 9. Dewaldt Duvenage, 8. Jebb Sinclair, 7. Rynhardt Elstadt, 6. Siya Kolisi, 5. De Kock Steenkamp, 4. Eben Etzebeth, 3. Frans Malherbe, 2. Tiaan Liebenberg, 1. Deon Carstens
Substitutes: 16. Deon Fourie, 17 Brok Harris, 18. Quinn Roux, 19. Don Armand, 20. Nick Fenton-Wells, 21. Louis Schreuder, 22. Burton Francis