At the time of Chanukah, was there still a Sanhedrin practicing in the Beis Hamikdash, or were they in exile? If they were in exile, does the Rav know when they would have returned?
How long after the neis of Chanukah was the chag set down, and the brachos enacted?
We may assume that if the Greeks were ransacking the Mikdash, no Sanhedrin was present. They would have been in hiding. There is no reason to assume that they didn't return immediately upon regaining control of the Mikdash area. And even before that time, they may have passed Halachic rulings from their place of hiding.
Megilas Ta'anis states that the holiday of Chanukah was instituted the following year, i.e. on the first 25 Kislev to occur after the miracle itself.