A modification of my thoughts on Persia
I think Persia might have recovered a bit faster than i first thought. If you look at their final battle vs the Arab invasion they put a very organized force in the field.
Battle of al-Qādisiyyah https://en.wikipedia.org/wiki/Battle_of_al-Qādisiyyah
Sassanid Persia[edit]
The Persian army reached Qadisiyyah in July 636 and established their highly fortified camps on the eastern bank of the Ateeq river. There was a strong bridge over the Ateeq river, the only crossing to the main Sassanid camps, although they had boats available in reserve to cross the river.
The Sassanid Persian army, about 60,000 strong, fell into three main categories, infantry, heavy cavalry, and the Elephant corps. The Elephant corps was also known as the Indian corps, for the elephants were trained and brought from Persian provinces in India. On 16 November 636, the Sassanid army crossed over the west bank of Ateeq, and Rostam deployed his 45,000 infantry in four divisions, each about 150 meters apart from the other. 15,000 cavalry were divided among four divisions to be used as reserve for counter-attack and offensives. At Qadisiyyah, about 33 elephants were present, eight with each of the four divisions of army. The battle front was about 4 km long. The Sassanid Persians' right wing was commanded by Hormuzan, the right center by Jalinus, the rear guard by Piruzan, and the left wing by Mihran. Rostam himself was stationed at an elevated seat, shaded by a canopy, near the west bank of the river and behind the right center, where he enjoyed a wide view of the battlefield. By his side waved the Derafsh-e-Kāveyān (in Persian: درفش کاویان, the 'flag of Kāveh'), the standard of the Sassanid Persians. Rostam placed men at certain intervals between the battlefield and the Sassanid capital, Ctesiphon, to transmit information.
The problem is that they were still in the middle of a civil war - despite the arabs. Without the arabs they wont have any reason to put aside they differences so I expect it to continue and be worse.