Old Testament and New Testament Sabbath 1 (3)

In Hebrews 7:11, it reads, If perfection could have been attained through the Levitical priesthood( for on the base of it the law was given to the people) why was there still need for another priest to come-one in the order of Melchizedek, not in the order of Aaron?

 In the beginning of this verse, we see a condition, IF. The Levitical priesthood found in the Old Testament was given through theorder of Aaron , the high priest and the law was given to the people. 

However, if the Levitical priesthood was not perfect, then what happened? Another priest came not in the order of Aaron but in the orderof Melchizedek. Then, who came as the the high priest in the order ofMelchizedek?