R. Abin the Levite said: Whoever tries to force his [good] fortune will be dogged by [ill] fortune,4 and whoever forgoes his [good] fortune will postpone his [ill] fortune.5 This we can illustrate from the case of Rabbah and R. Joseph. For R. Joseph was 'Sinai'6 and Rabbah was 'an uprooter of mountains'.7 The time came when they were required [to be head of the Academy].8 They [the collegiates] sent there [to Palestine] to ask, As between 'Sinai' and an 'uprooter of mountains', which should have the preference? They sent answer: Sinai, because all require the owner of wheat.9 Nevertheless, R. Joseph would not accept the post, because the astrologers had told him that he would be head for only two years. Rabbah thereupon remained head for twenty-two years, and R. Joseph after him for two years and a half.10 During all the time that Rabbah was head, R. Joseph did not so much as summon a cupper to come to his house.11According to Rashi, this was that Rav Yosef never assumed any airs of office, and so he did not even summon the cupper, meaning the bloodletter, but rather went to him like an ordinary person. And so follow Soncino, Artscroll, Shteinsaltz.
Nobody even suggests the obvious alternative, which upsets me a bit.
Realize that Rav Yosef was told this by astrologers. He delayed this good fortune, and in this way delayed his ill fortune. He knew he would only be head of the yeshiva for two years. If he had assumed office immediately, then he would only be fated to have two years of life; or two years before having to step down from his position. However, now that he was assured of his position, he knew that he would live so long as Rabba was in office.
Therefore, so long as Rabba was in office, Rav Yosef did not need to fear for his health.
A blood-letter is summoned for health purposes.
But Rav Yosef had no need to summon the blood-letter, because he was confident in his health. Thus, this is not a lack of serara in summoning, but a confidence in his fate.