1. Tosfos on this Gemara (DH b'Uriah) explains this. Tosfos asks why does the verse in Yeshayah 8:2 and the Gemara here link Zecharyah and Uriah? After all, we find nowhere that the prophecy that Tzion will be plowed over like a field was said by Uriah, but rather we find in Michah 3:12 that it was said by Michah.

2. Tosfos answers that the matter may be understood if we look at chapter 26 of the Book of Yirmeyah, where Yirmeyah prophesied that Yerushalayim would be destroyed. The Kohanim and the Nevi'im of the time wanted to kill him for saying this, but the princes came to his defense and said that Michah had already made this prophecy and was not killed for doing so. They continued and said (Yirmeyah 26:20) that Uriah the son of Shemayahu from Kiryas Yearim also prophesied in the name of Hash-m about exactly the same things that Yirmeyah said.

3. Tosfos writes that since the princes said that Uriah also prophesied like Michah, this suggests that Uriah said exactly the same as Michah. Therefore, the verse in Yeshayah 8:2 and the Gemara here refer to it as what Uriah said, even though it is actually written in the Book of Michah.

4. However, even though Uriah also said it, is it not still surprising that the Gemara refers to this only as the prophecy of Uriah and seems to totally ignore the fact that Michah also said it and that it is written explicitly in his book? This question is asked by the Anaf Yosef, printed in the Ein Yakov. He answers that in fact the prophecy of Michah never actually materialized because his words made an impression on the nation, and they repented. Yirmeyah 26:19 states that as a result of Michah's prophecy, King Chizkiyahu prayed to Hash-m, and in response to his prayer Hash-m annulled the evil that He had planned to bring upon the nation. The Anaf Yosef writes that it follows that the "trustworthy witness" was in fact Uriah, because only his prophecy was actually carried out. In the times of Uriah, the people did not do repent and do Teshuvah, but on the contrary their behavior become even more corrupt, as Yirmeyah 26:21-23 relates; Uriah had to flee to Egypt, and King Yehoyakim sent emissaries after him who brought him back and murdered him. This is why the Gemara calls this the prophecy of Uriah even though it had been previously made by Michah.

5. The Teshuvos Tashbatz (2:88) elaborates on how the three prophecies of Michah, Uriah, and Yirmeyah were essentially the same. He writes that this is the way of the prophets: that even though the actual words they say are not the same, the content of their message is sometimes identical. This is what the Gemara in Sanhedrin (89a) means when it states that several prophets may say the same prophecy, but each prophet says it in a different way.

All the best,

D. Bloom