How did the damaged nerve in his left index finger impede Frank Marino's ability to perform?

He could no longer play like himself, impeding complex riffs and stretches

The damaged nerve in the index finger of his left hand presented a critical obstacle because it directly compromised his execution of his signature playing style. The text indicates that this injury meant he could no longer play 'like himself.' The pain and limited movement specifically impeded his ability to execute the familiar complex riffs and physical stretches that defined his virtuosity on the fretboard. This was a devastating blow because his musical identity was so intertwined with his improvisational prowess and technical command, leading him to even contemplate the drastic measure of learning to play left-handed before seeking specialist intervention.