Sheparding according to Nietzsche

After Nietzsche discusses the idea of who should be in charge of the sick, he comes to the conclusion that the "ascetic priest is the predestined savior, shepard, and advocate of the sick herd...his historical mission is dominion of the suffering in his kingdom...he must be sick himself...but he must also be strong, master of himself even more than of others...he has to defend his herd againsst the healty" (125-126). Nietzsche decides that the sick, actually the less-sick, but not the healthy, should be in charge of protecting their own sick kind. My only concern is how the less-sick, who are weaker than the strong, possess the strength to defend themselves against the strong. It seems to me that the shephard cannot actually defend his herd, that the predator is much stronger than its prey. The question now is, why does nietzsche advocate the less-sick as the shepard when it seems that there is no why in reality they can actually survive.