אתמר ר' יוחנן ור' יהושע בן לוי חד אמר ערבה יסוד נביאים וחד אמר ערבה מנהג נביאים תסתיים דר' יוחנן הוא דאמר יסוד נביאים דא"ר אבהו א"ר יוחנן ערבה יסוד נביאים הוא תסתיים א"ל ר' זירא לר' אבהו מי א"ר יוחנן הכי והא"ר יוחנן משום ר' נחוניא איש בקעת בית חורתן עשר נטיעות ערבה וניסוך המים הלכה למשה מסיני (דנייאל ד) אשתומם כשעה חדא ואמר שכחום וחזרו ויסדום ומי א"ר יוחנן הכי והא"ר יוחנן דלכון אמרי דלהון היא ל"ק כאן במקדש כאן בגבולין
Rav Yochanan says that the minhog of Hoshannos was forgotten and the nevi'im reinstated it. Rashi there says "ahl pi hadibbur." The Rashash and the Maharitz Chajas both express their perplexity at this Rashi, because the Gemara in Temura 16 (regarding Asni'el ben Knaz) clearly states that when halachos are forgotten, a navi cannot reinstate them on the basis of his prophecy because the Torah is not Bashamayim. Neither offers an answer that explains Rashi.
A member of my shiur, Rabbi Gary Schreiber, said a brilliant answer which I think is 'karov l'emes.'
The Nevi'im did not re-instate the halachah as a d'oraysa. They were told through Nevu'ah that they should institute a new din d'rabbanan that happens to replicate what was a din d'oraysa. Despite the fact that they, and we, knew that this had previously been a din d'oraysa, the rule of lo bashamayim mandated that it be treated as a din d'rabbanan forever after. This explains why Rav Yochanan referred to it as a Yesod Hanevi'im.
The great accomplishment of Asni'el ben Knaz was that through his pilpul, he was able to darshen the dinim that had been forgotten from the Torah itself with the rules of drasha, and therefore they remained d'oraysa, just like kidushei kesef is d'oraysa.
