WASHINGTON -- Ken Waldichuk firing 96 mph fastballs past big league hitters this early into his professional career, for the A’s no less, seemed impossible based on where he was just five years ago.

Undrafted in 2016 out of high school, Waldichuk went that entire summer without receiving a single scholarship offer from a Division I program. Exploring his limited options, he took note of how Saint Mary’s College had pitchers Corbin Burnes and Tony Gonsolin -- one the 2021 National League Cy Young Award winner, the other a 2022 All-Star -- selected in the 2016 MLB Draft. Waldichuk enrolled at the small school in Moraga, Calif., joining the baseball program as a walk-on.

Soon after, Waldichuk gained the fastball velocity to match his big 6-foot-4, 220-pound frame, and he dominated the West Coast Conference over three seasons, leading to the Yankees selecting him in the fifth round of the 2019 MLB Draft.

Once in New York’s system, Waldichuk quickly ascended, going from High-A to begin the 2021 season all the way to Triple-A by May 2022, establishing himself as the headlining prospect acquired by the A’s in exchange for Frankie Montas and Lou Trivino at this year’s Trade Deadline.

Even as he’s developed into a heralded talent who now ranks as Oakland’s top pitching prospect and the No. 70 overall prospect in baseball per MLB Pipeline, Waldichuk kept that underdog mindset he embraced when no Division I school felt he was good enough for a scholarship.

“I try to stay in that walk-on mentality,” Waldichuk said. “I just try to outwork everyone and just do whatever it takes to get results. It’s been awesome to see hard work pay off.”

The payoff of that hard work finally arrived Thursday in the form of his Major League debut. Taking the mound at Nationals Park in a 7-5 walk-off loss to the Nationals in 10 innings, Waldichuk showed off his tantalizing arsenal. Over 4 2/3 innings, the left-hander held Washington to one run on five hits and four walks while striking out six batters.

Combined with his battery mate in A’s No. 1 prospect Shea Langeliers, who homered in the fifth and drove in two runs with a booming double in the 10th, the pair showed a major glimpse of what could be the foundation of a very bright future in Oakland.

“I thought he did really good,” Langeliers said of Waldichuk. “He came out and was probably a little nervous. Right before, I told him, ‘Hey, just go out there and be yourself. You’re nasty.’”

Though he boasts a pair of plus secondary pitches with his changeup and curveball, as well as a sweepy slider with some tilting action, Waldichuk dazzled mostly with his heater. He threw the fastball for 53 of his 93 pitches, generating seven of his 15 total whiffs (swing and misses) with it and utilizing it as the putaway pitch to five of his punchouts.

“He’s got really good stuff,” said Langeliers, who previously caught Waldichuk three times at Triple-A Las Vegas. “He’s got an explosive fastball and changeup that just falls off the table. If he’s commanding his curveball and his slider, he’s nasty. He did a good job commanding the fastball and the changeup today, and [he] landed some curveballs and sliders.”

Waldichuk’s first start did not come without adversity. The Nationals took a 1-0 lead against him just two batters into the first inning, and he pitched with traffic on the bases in all but one inning. But it was in those stressful moments that Waldichuk seemed to elevate his game.

Following a leadoff triple by Alex Call in the second, Waldichuk retired the next three batters he faced, including two via strikeout. One inning later, Waldichuk pitched himself into and out of a bases-loaded jam, retiring Call on a flyout to keep Washington off the scoreboard.

“After that first, I felt a little more settled in,” Waldichuk said. “The nerves in the first were something I had to work around. It was a little rocky. There was a lot of traffic. But it was a pretty cool experience to have.”

Now that the first-game jitters are out of the way, the A’s plan on keeping Waldichuk in the rotation. With electric stuff that reminds some in the organization of former A’s lefty Sean Manaea in his earlier years, the goal is for Waldichuk to acclimate to pitching at the highest level. That way, once 2023 comes around, he’ll be ready to move into a role as a stalwart in Oakland’s starting rotation for the foreseeable future.

“Ken did a nice job for his first outing,” said A’s manager Mark Kotsay. “I liked the changeup. He started throwing that a little more as he got into the game. Going forward, it’s a learning curve for him. We’ll build off this one.”

Waldichuk wasn’t the only rookie who debuted. Cody Thomas, also called up from Triple-A Las Vegas on Thursday, got the start in left field and poked a single to left in the fifth for his first Major League hit. The debuting duo marked the 15th and 16th rookies to make their Major League debut this year, setting an Oakland record for the most in one season.