"This HURTS," the former Indiana Pacers star said on his social media page on Monday.

"The man who bought me my first set of baseball cleats, first basketball, who taught me to drown out all the outside noise and FOCUS is now and will forever be my guardian angel."

Miller's father, Saul, had five kids with his wife, Carrie -- including Reggie and Cheryl Miller, a former basketball star herself.

In his Instagram post, Reggie thanked his dad "for EVERYTHING" he's done for him and the Miller family.

"Say hello to Mom for us," the 57-year-old former shooting guard wrote, "know that your teachings will live on threw me to my kids."

"Please respect my brothers and sisters during this time," he added, "though difficult he’s in a much better place.. #PopGuardianAngel"

Saul grew up an athlete ... according to author Mark Montieth, he played hoops in high school and at Lemoyne College.

Reggie has spoken about the impact his dad had on his life and career before ... thanking him at length in his 2012 Hall of Fame speech.

"You made time to be there for all of us," Reggie said. "So, again, Dad, Mom, thank you so much for everything you've done for us."

Saul also served 26 years in the Air Force and was a jazz musician.