The Gemara wants to understand where Rava’s statement that “the word שה comes to exclude a crossbred animal” applies, and first says that his statement was made in regards to redeeming the donkey, whose holiness can not be transferred onto a crossbreed. The Gemara then asks that according to R’ Elazar who says that one can use a crosbred animal for this function, where would Rava’s statement apply? To this we say, “R’ Elazar would answer that Rava would apply in a case of a Kosher animal giving birth to an unkosher animal.”
Question is, why is R’ Elazar, who is a Tana and predates Rava, forced to explain Rava? If anything, Rava should have to explain himself according to R’ Elazar!
My guess would be that Rava was not saying this statement on his own, but rather, was quoting an ancient tradition that predated the Tannaim. The Gemara here itself seems to be assuming that indeed Rava’s statement was powerful enough to warrant extensive discussion as to where it applies. The fact that R’ Elazar has to reckon with Rava’s statement strongly indicates that it was an old enough statement that R’ Elazar too should have known of it, and therefore the Gemara has R’ Elazar answering ‘according to Rava.’
בכל מקרה, צריך עיון יותר ומקור לכזה פשט