Heartbroken Over Mills Lane's Death

Legendary boxing ring announcer Michael Buffer says he's heartbroken over the passing of Mills Lane ... telling TMZ Sports the fight ref was "one of the all-time greats."

Lane passed away on Tuesday morning ... after he had been in hospice. His son, Tommy Lane, told the Reno Gazette Journal that Mills had taken "a significant decline in his overall situation" recently and died while surrounded by his family.

Just days after his passing, Buffer remembered Mills as a solid dude who he'd miss.

"I worked with him many times," Buffer said at LAX. "For me, I always just loved the way he would say, 'Let's get it on!' through the instructions in the middle of the ring."

"He was a great guy."

Buffer also…