"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 like Josh Koscheck, Kenny Florian and Forrest Griffin among others.
Bonnar's finale fight with Griffin is one of the most famous fights in the promotion's history.
"[Stephan] worked my corner for one of my UFC fights. We were brothers. We stayed in contact and he was doing good, man. He was finally turning the curve," Sanchez said of his friend who at times struggled to find his way after his fighting career ended.
But, Diego -- who was cornered by Stephan at UFC 253 -- says his friend was on the right path.
"As a big brother who's been through the rough, the rain, the pours of life and he got his umbrella and he was coming out and he was going into the light. He was going into the sunshine."
Diego continued ... "He had his podcast, he had a new job with an insurance company and he was just working on rebuilding his life. I was happy for him and excited for him."
Bonnar's cause of death has not been announced, but earlier reports indicated he died from heart failure while at work. Stephan is survived by his wife Andrea and young son, Griffin.