On the bottom of 41b the Gemorah tries to prove that Rav Yosef's explanation of the Rebbi-Rebanan machlokes is correct, and not Rabbah's version, because according to him the braisa that the gemorah quotes fits nicely according to Rebbi, but according to Rabbah the Raisha fits well for both Rebbi and the Rebanan.
The gemorah's two answers are, seemingly, obvious. What was the question to begin with? What was the Haveh Amina that the fact that the Raisha could fit for the Rebanan as well, somehow detracts from the possibility that the entire Braisa is Rebbi? (i.e. the second answer of the Gemorah).
The KIKAYON D'YONAH asks your question.
He answers that the Gemara understood from the wording of the Beraisa that the original Beraisa only included the Reisha. If so, it was unlikely that Rebbi would add to the Beraisa a Halachah which did not fit perfectly with the original Beraisa, i.e. the Reisha.
P.S. Sorry for the delay due to the Yom Tov break.