Butch -- real name Bob Miller -- was hospitalized in the ICU on Friday after arriving in Los Angeles for WrestleMania ... which was a result of a "situation with medication," Luke said in an interview over the weekend.

"He flew in from New Zealand on Wednesday, was all good. Friday morning, he was incognito."

Butch started wrestling in New Zealand in 1964 ... and made his way to the States shortly after. Alongside Luke, the Bushwhackers blew up once they signed with the World Wrestling Federation in the late '80s.

Butch was an absolute legend for decades ... and was inducted into the WWE Hall of Fame in 2015 as a member of the Bushwhackers.

Luke released an emotional tribute to Butch just minutes ago ... saying, "Late last night, I lost my friend, brother and tag team partner of over 50 years with the passing of Bob 'Butch' Miller."

"From the early-1970s when we were young mates wrestling for John da Silva in New Zealand, my first impression of Bob 'The Chest' Miller (as he was called in those days) was that he was a first-class redneck, and what bloody redneck he was! But he was also an all-around good guy and a great friend."

"If you were to read the story of my life it would say that I am only child, but it would be wrong," Luke added. "In life I had a brother and his name was Bob Miller. I love you, Bob. Until I see you again, WOOOOAHH YEEEEAAHH!"

A GoFundMe page was set up to help with Butch's medical expenses ... and the funds will now be used to pay for a funeral service.

Miller is survived by his wife, Helen, daughters, Sharon and Kirsten, and his grandchildren.