During the early days of the Muslim conquest astonishing gains were made by Arab armies who managed astounding victories against the two powers of the time. The Persian empire was completely conquered while the Roman was greatly weakened but continued to fight on for centuries after. Would it be possible to reverse this? that is can we have the Romans completely roll over while the Sassanids lose substantial territory but remain as a rump? Say the Sassanids lose Mesopotamia but manage to stop the Arabs from taking Iran. What would the effects of this happening be?
There could be so many effects.
If the entire Byzantine Empire falls, including all the exarchates in the West, then the Germano-Roman kingdoms would come under pressure very early, at a time when the Avars are exerting pressure, too.
Since this is pretty early in the development of Islam, a changed self-concept of Islam, one which sees it as the true revival of the best of Rome, one which would merge the Caliphate and the title of Roman Emperor, is conceivable.
But it´s not inevitable; the maintenance of the Arab character of Islam is equally likely.
In Sassanid history, there hasn`t been a time when lost parts of the empire´s core were simply given lost; there were frequent, repeated attempts at reconquering such territories. The Sassanids as we know them would wear themselves down in repeated fights against the overwhelmingly strong Muslim armies.
Different Sassanids, on whom the event leaves a lasting imprint, would be left with a rather mountainous territory where centralized rule, one big Sassanid fetish, would be difficult. Their biggest asset would still be control over the Silk Road, the bridge between China and India, and, for that matter, also between China and the Muslim Mediterranean. In the mountainous terrain, the social structure of the Sassanid Empire, based on a stratified land-owning nobility, would enter a serious crisis: much less agriculturally productive land, much more pastures where nomads usually dwell. The geography of an empire in the Zagros, Elbrus and Hindu Kush mountains is one much more prone to tribalism than to centralized urban government. Can the Sassanids adapt and find a new balance between decentralisation, coordinated defense and stable control over the important trade routes?