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Bad news, Raiders fans ... Justin Gatlin says he suffered a similar cryotherapy injury as star receiver Antonio Brown -- and the U.S. sprinter says it took him MONTHS to get healthy.

The superstar Olympian famously got frostbite on both his feet and ankles after a mishap in the cryogenic chamber back in 2011 -- and the guy says the injury hurt like hell.

In fact, when Gatlin joined "TMZ Sports" (weeknights on FS1) on Wednesday, he said of the frostbite, "It was a disaster. I had, like, boils and blisters all over my ankles."

Gatlin says the injuries were similar to the ones Brown posted on social media a couple weeks ago ... and says he thinks it could be a while before AB is healthy again.

"He's not going to be able to run those routes properly without healing his feet totally."

Gatlin says the recovery process for the injury was brutal, telling the guys, "Every day I had to get those blisters popped by hand by a therapist ... It just got really, really bad."

Good news for Brown ... Gatlin says AB should still be able to resume cryotherapy treatments once he's all healed up by using a more localized form of the cold stuff.

As for AB, the Raiders have been mum on a timeline for the stud wideout ... only calling him day-to-day.

Gatlin says he's friendly with Brown and would be more than willing to offer the WR advice on how to treat the injury.