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"Mean" Gene Okerlund was seriously injured during a nasty fall back in December ... triggering a rapid decline in his health that ultimately led to his death, his son tells TMZ Sports.

We spoke with Todd Okerlund who says Okerlund suffered multiple broken ribs during the fall at his Florida home and was treated at a hospital in Sarasota.

Todd says Okerlund had been in frail health for a while but the fall clearly made things much worse.

Just 4 days before his death, Gene was admitted to a nursing home .... but he began to have trouble breathing and was rushed to a hospital on Wednesday where he ultimately passed away with his wife, Jeanne, by his side.

Todd tells us ... the family has been touched by all of the support for his father. Todd also says Gene LOVED his career and would have done it for free.

"Mean" Gene is a WWE Hall of Famer who was a key part of the company's success -- with legendary interviews with Hulk Hogan, Andre the Giant, "Macho Man" Randy Savage and more.

Vince McMahon also issued a statement on Okerlund ... saying, "It was impossible not to crack a smile whenever 'Mean' Gene Okerlund entered a room. He was the voice behind so many of WWE’s most iconic and entertaining moments, and the WWE family will miss him immensely."

Gene was 76.