Diego Sanchez Stephan Bonnar's Death 'Truly Devastating' ... Both Starred On 'TUF1'

"I was on my knees crying. I loved Stephan."

UFC legend Diego Sanchez says he was crushed by the sudden death of his longtime friend and The Ultimate Fighter 1 star Stephan Bonnar ... who passed away unexpectedly on December 22.

"It was devastating. It was truly devastating. I was halfway through Christmas Eve just spending time with my family and I had someone text me, tried calling me, and then they texted me, 'You really wanna take this call.' I called them back and they gave me the news," Sanchez told us.

Diego and Stephan were both castmates on TUF1 -- which aired in early 2005 -- and are credited with helping the promotion grow into the biggest in all MMA. The season produced fighters…