Albert Barnes on Hebrews 7:15

And it is yet far more evident. Not that our Lord would spring out of Judah, but the point which he was endeavouring to establish; that there must be a change of the priesthood, was rendered still more evident from another consideration. A strong proof of the necessity of such a change of the priesthood was furnished from the fact that the Messiah was to be of the tribe of Judah; but a much stronger because, as a priest, he was to be of the order of Melchizedek--that is, he was of the same rank with one who did not even belong to that tribe.

After the similitude, Resembling; that is, he was to be of the order of Melchizedek.