"It was scary," the now 32-year-old Johnson said. "My first fear was that I couldn't play baseball anymore. Baseball and basketball were my two favorite things to do. That was tough, as a kid."
Johnson also knew the dangers of the disease. His dad's sister had died from diabetes complications.
The young Johnson needed encouragement. He needed someone to tell him he could manage the disease, and his dreams of becoming a pro ballplayer could still come true.
That's where his father, John, came in.
"My dad said, 'Don't let this stop you from doing anything you want to,'" Johnson remembered. "'If you love to play sports, keep doing it. You're not going to be any different than anyone else when it comes to that.' That always stuck with me."
It stuck with Johnson through all the insulin shots, all the blood sugar readings and all the steps that have led him to be a veteran Major Leaguer pitching in the Indians rotation this season.
To any child with diabetes, Johnson is an inspiration. Because from the day he found out about the disease, he hasn't let it get in the way of his development as an athlete.
"At the time I found out, I didn't know anything about it," Johnson said. "I was completely clueless about it. I didn't know what I could do or couldn't do."
At 11, Johnson had all the normal symptoms of diabetes. He was constantly dehydrated and often fatigued. Tests revealed what his family suspected.
"Normal blood sugar is around 80 to 120 [milligrams per deciliter]," Johnson said. "Mine was around 490."
Though the diagnosis was nerve-wracking, Johnson's doctor encouraged him to be a normal kid. Yes, he would have to check his blood sugar often and watch what he ate and drank, but he could still play sports.
As his skills blossomed in high school in Hebron, Ky., Johnson began to catch the eye of pro scouts. Some coaches and fellow players were skeptical about the chances of him making it in baseball with diabetes, but he never let negativity get the best of him.
The Pirates took a chance on Johnson by drafting him out of high school at the age of 17. Three years later, he made his big-league debut in Pittsburgh.
Johnson's career has since taken him to the Devil Rays, Orioles, Tigers and, now, the Tribe, where he'll be the fifth starter on a club hoping to compete for the AL Central title. Though he's a proven innings-eater, the knock on the right-handed Johnson is that he's won just 52 games in 199 starts.
Then again, he's never been on a successful team, and the hope is the Indians' offense will improve his chances of compiling a winning record.
"When you're on a team that loses a lot, it's kind of hard to get up for every start," Johnson said. "I always went out there 100 percent ready. But when you give up a couple runs, and we wouldn't be able to score, it kind of became a habit. So being here should help me this year."
That he is here at all is a testament to Johnson's strong will in the face of tough odds.
Not only has he refused to let diabetes get in his way, he's found a way to revolutionize how the disease is treated while he pitches.
Johnson used to have to endure three injections of insulin every day, from the day he was diagnosed with diabetes until he was 28 years old. Often, he would have bad reactions and low glucose levels at night, while sleeping.
Needless to say, such reactions were quite troublesome for Johnson and his wife, Stacey. So four years ago, Stacey encouraged Jason to begin using an insulin pump.
"She studied it and said maybe I should check on it," Johnson said. "She takes care of me. I probably wouldn't be here without her. She wanted me to try it out."
One problem, though. Major League rules prohibited Johnson from wearing the pump while he was on the mound.
Seeking to change that rule, Johnson sought permission from the league to wear the pump before the 2004 season with the Tigers. He had the Tigers' trainers take pictures of the device, which is about the size of a pager, and videotape him while pitching with it.
"[The league] really wanted to know what it would look like," Johnson said. "They found it was perfectly fine."
The pump keeps Johnson from having to prick his fingers several times a game to check his glucose levels, and it's also helped his physical endurance on the mound. It's not a distraction to opposing batters, and it's really only noticeable to fans if one makes it a point to look for it.
In fact, while wearing the pump, Johnson almost looks as if he's hooked up with a microphone for TV.
"A lot of people think I'm mic'd up," Johnson said with a laugh. "A lot of people still don't have a clue what it is yet. The pump hasn't bothered me at all. It's on the back of my belt. I know it's there, but it really doesn't affect me at all."
Nor has Johnson let the disease affect his career. That's a message he's spread to children through his work with the Juvenile Diabetes Research Foundation(www.jdrf.org).
Johnson and the JDRF teamed up last September for an event at Comerica Park that raised $60,000 for diabetes research. He said he plans to work with the non-profit organization's Cleveland chapter this year.
None of this charity work would have been possible if Johnson didn't receive the encouragement he needed at the age of 11. He's taken it upon himself to provide such encouragement to young children with diabetes.
In fact, Johnson relishes the role.
"It's big," he said. "It's very important to me. For kids to be able to look at me and say, 'He's in the Major Leagues with the same disease I have,' it encourages me to do things for the better, whether it's sports or anything in life. That's all that matters to me. If they realize they can do it because I am, that's my purpose of being a Major Leaguer with diabetes."