Yeah, the Sassanids were in a terrible position, actually. If you get them to rest for a bit, for whatever reason, like maybe they're preparing for a war against the Byzantines, but the emperor dies, so the preparations are delayed until, coincidentally, the Arabs come along.
The Sassanids prepared for the war with the Byzantines under Khosrau and even had the Avars as allies, yet still were bested even with all the cards in their hand. At this point in time, the Sassanids were quite weak compared to their early days. Even if prepared, as the Sassanids should've been, they would've been pushed back by the superior experience and generalship of the Arab armies who have been waging war in Arabia for at least a decade gaining phenomenal experience under fantastic generals and individual fighters like Ali ibn Talib. The Sassanids had time to prepare and did, but where still routed and dismantled, just as they were against Heraclius who had logistical problems, the result with an enemy more fearsome than Byzantium and less logistic issues ended in the complete dissolution and destruction of the Sassanid state.
This is a clear issue and fact. The Sassanids had years in advance to prepare and did, yet were defeated while the Rashidun looked at Byzantium. The early Sassanids are a different matter, but the Sassanids of the 600s where not what they used to be and not in position to transition into the new era.
An interesting question is which is more impressive, Heraclius fighting off the Avars-Sassanids or the same against the Caliphate. Very interesting.