Explanation #1: Rashi rejects the text 'Amar Ravina' ...

Reason: Since he and Rav Papa are not arguing.

Query: This is not however, the way of the Gemara (see Maharam).

Explanation #2 (Part 1): Rabeinu Tam therefore explains that Ravina actually comes to resolve the entire Mishnah - inasmuch as 'ha'Meisis Zeh Hedyot' (the Reisha) implies that a Hedyot is Meisis (and does not incorporate a Navi as well). Whereas 'ha'Meisis es ha'Hedyot' (the Seifa) implies anyone who is Meisis, even a Navi.

Explanation #2 (Part 2): Whereas Rav Papa holds that 'ha'Meisis es ha'Hedyot' does not imply even if a Navi is Meisis.

Observation: The Gemara at the beginning of Yevamos extrapolates from here that R. Yehudah does not Semuchin on principle.

Question (Part 1): Perhaps it is only here that he does not, because there is a Gezeirah-Shavah which negates the Semuchin (i.e. R. Yossi ha'Gelili, who learns "Lo Sechayeh" from "Lo Sechayeh Kol Neshamah" by Sayaf")?

Proof: Will we say that R. Akiva, who learns from "Lo Sechayeh" of Sekilah, does not hold like the Rabbanan who Darshen Semuchin?

Question (Part 2): We are therefore fored to say it is the Gezeirah-Shavah of "Lo Sechayeh" that precludes Semuchin.

Answer: It is from the Lashon of R. Yehudah 've'Chi Mipnei she'Samchu ... ' (which implies that he does not hold of Semuchin) that the Gemara in Yevamos draws its inference.