Michael Moorer -- one of the few men who stood toe-to-toe in a boxing ring with George Foreman -- tells TMZ Sports he's heartbroken over the boxing legend's death ... making it clear the pugilist will be sorely missed.

We sat down with Moorer on Monday -- just three days after Foreman's family revealed the ex-heavyweight champ passed away at 76 years old -- and it wasn't hard to tell he was crushed to learn of the news.

Moorer called Foreman "a gentle giant" ... and added that he'll always remember him as "a great person."

"It was shocking," the former fighter said of George's death. "I didn't believe it was true at first ... It was devastating."

Moorer and Foreman fought way back in 1994 -- and Moorer was considered a big favorite at the time. And, for much of the matchup, he dominated ... but in the 10th round, Foreman knocked him out with a huge right hand and took his championship belt.

Moorer -- who said he needed 32 stitches in his lip to fix the damage the KO punch caused -- held no ill-will toward Foreman in wake of the defeat ... in fact, he said the two remained telephone friends over the years.

He got the chance to catch up with Foreman at a 2023 screening of the "Big George Foreman" biopic ... and told us he was thrilled his son got the chance to snap a photo with the legend while at the event.

Foreman's family stated he died "peacefully" while "surrounded by loved ones." A cause of death has not yet been revealed.