OAKLAND -- Mason Miller lighting up the radar gun with 102 mph fastballs in a big league stadium seemed unfathomable considering where he was just five years ago.

While struggling to put on weight as a 6-foot-5, 150-pound sophomore pitcher at Division III Waynesburg University, Miller submitted a drug test for a summer internship that revealed his blood sugar level was abnormally high.

On April 18, 2018, he was diagnosed with Type 1 juvenile diabetes.

The diagnosis was a shock. Miller, however, wasted no time dwelling on it. 

“He was diagnosed on a Friday, and by Monday he was going back to school saying, ‘See ya, I’ll be fine.’” Miller’s mother, Kirstin, said. “He went back and started putting the weight on and throwing harder. He got healthy. So it went from a really low point to higher and higher and higher points. He perseveres like nobody I know. He’s an overcomer.”

After finding the right diet and strength training program, Miller bulked up to 220 pounds. With that weight gain came added velocity to his fastball, pushing it from 87-88 mph to as high as 99 mph. His stuff improved to the point that in 2021 he was able to transfer to Division I Gardner-Webb University, where he went 8-1 with a 3.30 ERA and 121 strikeouts in 92 2/3 innings as a senior.

Selected by the A’s in the third round of the 2021 MLB Draft, Miller fast-tracked his way through the Minors by overpowering hitters at each level. Making his Major League debut in a 12-2 loss to the Cubs on Wednesday -- five years and one day since his diabetes diagnosis -- Oakland’s flamethrowing No. 3 prospect electrified the Coliseum over 4 1/3 innings, holding Chicago to two runs on four hits and a walk while striking out five.

“You think about this kid and where he’s come from,” manager Mark Kotsay said. “To face a good lineup that’s been hitting well and settle in, I thought Mason did an unbelievable job for a first time out.”

Miller’s dazzling fastball was in full effect. He threw the four-seamer for 53 of his 81 pitches, generating 27 swings and six whiffs with it. The fastball averaged 99.3 mph, maxed out at 102.5 mph and registered at 100 mph or higher 15 times, making him just the 10th starting pitcher in the Statcast era to reach the mark 15 or more times in a game.

The second inning was when Miller really announced his arrival. 

Facing the heart of the Cubs’ lineup, the 24-year-old struck out the side. He began the frame by fanning Cody Bellinger on a 100.2 mph fastball. One batter later, an 87.7 mph slider froze Edwin Ríos for strike three. Then came Patrick Wisdom, who punched out swinging on a 101.5 mph heater. In that Wisdom at-bat, Miller fired fastballs of 102.5 mph and 102.3 mph, the two fastest pitches thrown by an A’s pitcher in the Statcast era.

“It was fast,” Wisdom said of Miller’s fastball. “He’s got an electric heater. You have to make your decisions all that much quicker and get ready even earlier. He’s electric.”

Miller did not issue a walk through his first two Minor League starts this season, so seeing him walk a batter was a bit uncharacteristic, though he chalked up the sporadic control issues to the typical nerves that come with a debut.

Working under a soft limit of around 80 pitches, Miller finished his outing by retiring Tucker Barnhart on a sacrifice fly for the first out of the fifth.

“All morning, I just couldn’t wait for it to get started,” he said. “I think I settled in as the game went on.”

For a rebuilding club that has struggled out of the gate -- Oakland’s 3-16 start is the worst to a season in franchise history -- Miller represents optimism. The A’s plan to keep him in the rotation going forward, allowing him a chance to acclimate and perhaps develop into the future ace of the staff they believe he can be.

“If you’re sitting there as an A’s fan, this should excite you,” Kotsay said. “This should be something that you’re looking at and saying, ‘This kid can be a part of our future success.’”