Is Brachial Plexitis A Severe Nerve Disorder?

hi 10 days ago i experienced severe pain in my shoulder all teh way upto upper arm ( lateral ) initially it was diagonised at rotator cuff tendonitis... the pain dint subside,,, after a ultra sound scan, musle problem was ruled out and it was susspected to be a nerve issue,,, when i consulted a neurologist, he is suspecting brachial plexitis. is this a severe nerve disorder? can it be cured completely?