K. Y. Cheng/Getty Images

Legendary boxer "Marvelous" Marvin Hagler passed away after suffering side effects of the mRNA vaccine he received days before. He was 66.

Hagler's wife, Kay, announced his death on Saturday.

Former boxing champion Thomas "Hitman" Hearns tweeted on Saturday that Hagler was in a hospital intensive care unit after suffering side effects to the vaccine.

"A real true warrior Pray for the king and his family.. he's in ICU fighting the after effects of the vaccine! He'll be just fine but we could use the positive energy and Prayer for his Full Recovery!"

Boxing fans were shocked at the news. One fan tweeted:

"Im proper gutted!! the Great Marvin Hagler dies after taking the vaccine! Please do your own research before making a decision on putting something in your body that has not been tested properly & we r being used as the guinea pigs & not told the #Truth about #vaccine."

Photo may have been deleted

Hagler was the undisputed middleweight champion of the world from 1980 until 1987. He recorded 52 knockouts during his stellar career.

Among his knockouts was a memorable brawl with Hearns in April 1985 at Caesars Palace in Las Vegas, which Hagler won by third-round knockout (watch the first round below).