Isaiah was not making a "Messianic prophecy" at all, he is talking of an incoming King in Israel at the time which he wrote.
I have not made a definitive study of "Isaiah", but I think little can be construed as Messianic, without retro-fitting the idea, as the Gospel Writers did.
Anything that may be vaguely Messianic would have been inserted by the Compilers of the Hebrew Bible, who worked after the Babylonian Exile, editing, redacting and generally altering the old Texts they did use, probably inventing whole chunks of new content themselves.