What would it take for the Byzantine Empire was conquered by the Islamic armies for the Sassanids to take a harsh beating but still end up surviving with a significant portion of their territory?
I'm aware that the original plans of Caliph Umar was to get a peace treaty from the Persians with the presumption that they were too powerful a foe to subdue and it was best to end any military engagements but the reigning Shah wasn't savvy with having one of the empire's richest provinces snatched by a bunch of desert nomads and ordered raids, which eventually pissed off the Arabs and sent them into conquering the rest of Persia.
So what if the Persian king decides to not bother attacking the Muslims, maybe bunker down and build fortifications along the Zagros, to protect the border from future attack? He'd be unpopular with the nobility who might prefer an offensive strike leading to an interesting political situation, maybe inviting Turkic tribes as mercenaries to protect him from the good chance of being overthrown by some ambitious satrap?
The effect of this could have the Nestorian Christians, concentrated in Mesopotamia and not allowed to convert in Islamic lands, to convert the Persian masses to Nestorian Christianity. Maybe a future Persian shah pulls a Constantine and embraces Nestorianism or will there be reforms to Zoroastrianism to avoid it being sidelined?