R. Akiva states (Y'vamos 8:4) that there is a distinction between the levirate obligations of an impotent man who has been that way all his life and an impotent man who became that way during his lifetime. Namely, the latter has a levirate status (which he is required to sever, but that's beside the point) because there was a time that he could father children and the former doesn't because he was never in a physical state conducive to fulfillment of yibum.
Rava explains (79b) that it need not be that the man is physically qualified right now, just that he was ever qualified. This is based on the fact that the same qualifications apply to both the deceased, who triggers the obligation, and the surviving brother, who fulfills it. And every deceased person had some period of time immediately before death in which they were physically unqualified to conceive children, due to the circumstances that near-death entails. Disqualifying such people would mean ineffectuating the mitzva of yibum, so it can't be what the Tora had in mind.
Why does R. Akiva's opinion allow a s'ris adam on the grounds of having had a sh'as kosher at an irrelevant time in the past before he was married? It could have allowed only those sarisim who had a sh'as kosher at a relevant time, thus excluding a yavam who had no sh'as kosher while his zika was active.