Former NFL speedster Chris Johnson once made defenders look frozen in place.

Now, the former Tennessee Titans star is facing a heartbreaking battle no one saw coming.

Johnson, a three-time Pro Bowl running back and one of the most explosive players of his generation, revealed Monday on Good Morning America that he has been diagnosed with ALS, the devastating disease also known as Lou Gehrig’s disease.

He was diagnosed last year at just 39 years old.

“There’s no history of ALS in my family,” Johnson told GMA co-anchor Michael Strahan. “My doctors believe my case is what’s called sporadic ALS, which is actually how the vast majority of ALS cases happen.”

Johnson said that is part of what makes the disease so terrifying.

“That’s one of the reasons this disease can be so shocking,” he said. “It can happen to someone who never expected it.”

For Johnson, the first sign seemed small.

He was still working out every day, spending time with his wife, Brittany, and their four children, and living what he described as the prime of his life. Then he noticed something wrong with his right hand.

“I first noticed weakness in my right hand,” Johnson said. “At first, it was little things like my grip didn’t feel right and I wasn’t as strong as I’ve always been.”

His wife thought it had to be tied to football.

After all, Johnson spent years taking hits in the NFL, racking up more than 11,000 total offensive yards before retiring in 2017. A pinched nerve or an old injury seemed far more likely than a life-changing diagnosis.

“I thought because of football and, you know, his career, that it had to be something with that,” Brittany told Strahan. “Maybe … a pinched nerve or something along those lines, but never ALS.”

But the answer was far worse.

ALS is a progressive neurological disease that attacks the nerve cells that control movement. Over time, it can rob a person of the ability to walk, talk, swallow, move, and eventually breathe.

There is currently no known cure. Some treatments may help slow the disease or improve quality of life, but ALS remains one of the most feared diagnoses a person can receive.

Johnson said hearing the words from his doctor was a crushing shock. He said he was told one medication might extend his life by only a few months, and that he and Brittany should “get our affairs in order.”

“Honestly, I don’t know if you ever fully process it,” Johnson said. “At first, you’re in shock. Then you realize you have two choices. You can give up, or you can fight. I chose to fight.”

That fight took a major turn after Johnson saw a Good Morning America interview featuring the late actor Eric Dane, who also battled ALS, and Dr. Merit Cudkowicz, a leading ALS researcher.

“After watching Good Morning America and seeing Dr. Merit with Eric Dane, we reached out to her,” Johnson said. “She was willing to think more creatively, offering experimental treatments that might help and advance research.”

“We’ve been with her ever since,” he added, “and that changed the course of our fight.”

Dr. Cudkowicz, a professor of neurology at Harvard Medical School and executive director of the Mass General Brigham Neuroscience Institute, said Johnson has been receiving standard care, including medications that can slow the disease. She also said he joined a clinical trial involving a therapy designed to reduce inflammation.

“I think that helped him a lot,” she told GMA.

Still, Johnson’s condition has moved frighteningly fast.

The man once known as “CJ2K” for his blazing 2,000-yard rushing season now uses a speech-generating device to communicate. His voice was recorded soon after his diagnosis, allowing the device to speak in a voice that sounds like his own.

“It’s continued to progress much faster than I ever imagined,” Johnson said. “I want people to understand just how quickly ALS can attack your body.”

Then he shared the kind of detail no parent wants to say out loud.

“Just over a year ago, I was picking up my 7-year-old daughter so she’d make a wish with her birthday cake,” he said. “Today, I couldn’t do that.”

Through it all, Johnson says his wife and children have become his reason to keep pushing.

“She hasn’t left my side through any of this,” he said of Brittany. “My kids are also a huge part of why I keep going. Every day I wake up wanting more time with them to make more memories and just be their dad. They give me a reason to keep fighting.”

Brittany said their life has changed completely, but she has no doubt about standing by her husband.

“Our life has shifted so much. It’s a heavy workload,” she said. “But I have no doubt that this is what, you know, I was called to do.”

Now the Johnson family is going public for a reason.

They want more people to understand how quickly ALS can strike, how desperately research is needed, and how important it is to find better treatments for families facing the same nightmare.

An effort supporting ALS research in Johnson’s honor has been established at the Sean M. Healey & AMG Center for ALS, led by Dr. Cudkowicz.

“I can’t even hold a cup if I try, and that’s despite being diagnosed relatively early and doing everything we can, including participating in multiple experimental treatments,” Johnson said. “That’s why early detection, more research, and better treatments are so important. We have to give people a better chance than what’s available today.”

But even as ALS changes what his body can do, Johnson wants fans to remember one thing.

He is still Chris Johnson.

“I want people to know that I’m still me,” he said. “ALS has changed what my body can do, but it hasn’t changed who I am.”

“People sometimes look at the physical disability and assume you’re not still the same person inside,” he added. “I still think the same. I still dream. I still love my family. My body just doesn’t cooperate.”