It seems that most meforshim (like Rashi, Radak, Ibn Ezra, Mahari Kara, Malbim) explain Isaiah 53 as referring to Klal Yisroel (or at least the righteous remnant of Klal Yisroel) and not the Moshiach. Why then does the Midrash in Ruth Rabbah 5:6 prove that Moshiach will have yisurin from the words in Isaiah 53:5, "והוא מחלל מפשעינו"?