In todays daf the gemara talks about sgan hakohen implying that the sgan would take over should the kohen gadol becomes tamei
In the first mishna I talks about cohen aher and rashi says that cohen aher was not the sgan since the sgan refuse to do it for reasons that when the cohen gadol became tahor again the the substitute would not be a cohen hediot would not either be a kohen gadol.
Shlomo amar, SA
I could not find the Rashi to which you refer. However, with regard to whether or not the Segan served as the Yom Kipur backup, we wrote in a recent discussion that it might be a Machlokes between the various Rishonim. (I am including a copy of that mailing below.)
If the Segan does replace the Kohen Gadol on Yom Kipur, you are correct in asking why the Mishnah on 2a does not mention this. The Gevuras Ari here asks the question, and answers that the Mishnah is discussing a situation where there was no Segan.
Re: Yoma 012: Segan Kohen Gadol
The Kollel replied:
>>As we discussed once in this forum (See Discussion of the Daf for Yoma 2), this was not normally the Segan Kohen Gadol. According to what you pointed out, the Segan was probably too busy to be used as a standby Kohen Gadol for Yom Kipur - after all, even on Yom Kipur his administrative duties would be needed.<<
David Retter asked:
Dear Rav Mordechai shlita:
In your reply, you state that the standby Kohen Gadol was not normally the Segan Kohen Gadol. Please note that the Bartenura in Sanhedrin Perek 2, Mishnah 1 appears to say that the two were one and the same. Also see the Tifereth Yisrael on the same mishnah.
The Kollel replies:
Excellent point, David, thank you! Actually, the source for the Bartenura is Rashi in Ta'anis 31a, and Tosfos in Nazir 47b (DH Mashu'ach) - see also Me'iri in Sanhedrin there, who also follows the opinion that the Segan was only needed to replace the Kohen Gadol on Yom Kipur. This must be the intention of the Gevuras Ari (Yoma 39a, cited in previous discussion) as well.
However, since Rashi (39a and elsewhere) and many Rishonim do not mention Yom Kipur, and since the Mishnah (2a) does not mention Segan, I inferred that other Rishonim hold the Segan was not the "Yom Kipur replacement". This may depend on the Machlokes (7b) whether the Tzitz is Mechaper while it is not being worn.