Sforno on Leviticus 13:5/ספורנו על ויקרא י״ג:ה׳

1וראהו הכהן, it is a decree by the Torah that only a priest is qualified to determine what is ritually pure and what is ritually impure in all matters pertaining to these skin afflictions. The prophet Maleachi 2,7 echoes this concept when he says that כי שפתי כהן ישמרו דת, “that the lips (pronouncements) of the priest will preserve the Jewish religion.” The priest is also required to counsel the afflicted person to examine his lifestyle so that he himself will become the key to his rehabilitation. At the same time the Priest will add his own prayer when asking G’d to heal the afflicted. Apart from his prayer his expertise will tell him when the stage has been reached when the affliction has waned so that the count toward rehabilitation and the presentation of offerings can begin.