זוהר בראשית — דף רכח ע”א

Jacob places Ephraim before Manasseh because Ephraim is spiritually greater. Joshua of Ephraim led Israel into the Promised Land. From Ephraim will come the Messiah son of Joseph who will precede the final redemption. When Jacob placed his right hand on Ephraim, he had in mind Joshua and all the great men of Ephraim. Jacob's blessing to Manasseh and Ephraim evokes the spiritual greatness of Ephraim and the messianic future.