Welp, that's awkward ... the cop who arrested Diego Sanchez last month told the UFC legend he was a longtime fan of the fighter ... admitting he watched him scrap since he was "a little kid!"
TMZ Sports obtained body camera footage of the arrest ... after cops alleged the 43-year-old shot a gun out of a moving car while officers were working an incident on I-40 in Albuquerque, NM.
Cops initially spoke to the driver of the car Sanchez was in .