OAKLAND -- When Mason Miller was shut down from throwing in May after he experienced right forearm tightness following his fourth Major League start, the A’s feared the worst for their prized young pitcher.

Even when the diagnosis came back as a mild ulnar collateral ligament sprain in Miller’s right elbow, manager Mark Kotsay and general manager David Forst expressed skepticism about a return this season. Miller, however, viewed the situation a bit differently.

“Trying to get back on that mound by the end of the year was my goal this whole time,” Miller said. “That’s what kept me going. After we got an initial look at it, the goal pretty quick was to try to get back this year and get some competitive innings before the end.”

Four months later, Miller -- ranked as Oakland’s top pitching prospect by MLB Pipeline -- found himself back on a big league mound, and he was just as nasty as before. Activated from the injured list prior to the A’s series finale against the Blue Jays, Miller took over for starter JP Sears in the sixth inning of a 5-2 victory on Wednesday afternoon at the Coliseum and dominated over two perfect innings of relief with three strikeouts.

The dazzling performance by Miller featured his blazing fastball, which he threw for 13 of his 27 pitches at an average velocity of 99.6 mph. Over the course of Miller’s outing, the four-seamer registered 100 mph or higher six times.

“Mason looked great,” Kotsay said. “The juices were definitely flowing. You could see it after that [sixth] inning when he came off the mound really fired up. It’s just a great sign. This kid, injuries have been a part of his career. To see him overcome it this year and get back on the mound with this type of performance, that says something about the character of the kid.”

After retiring three batters on a pair of popouts and a flyout in the sixth, Miller revved it up for his next inning.

The 25-year-old righty struck out the side on 15 pitches in the seventh, putting an exclamation point on his return by perfectly spotting a 102.4 mph fastball on the outside corner to Blue Jays slugger George Springer for a called strike three, marking Miller’s fastest pitch thrown for a strike in the Majors. He threw a 102.5 mph fastball that went for a ball in his Major League debut on April 19.

"He’s sensational to watch,” said Sears, who limited Toronto to one run on four hits and four walks across five innings. “Mason’s done a lot of work to get back where he is now, and I’m super hyped for him. … It’s good to have him be healthy and looking really good out there. Whenever you’re throwing 100 mph past guys, it’s going to be hard to hit. It was super fun to watch.”

The look of disbelief on Springer’s face really told the story. After watching the called strike three, he froze at home plate for a second before shaking his head as he made his way back to the visitors' dugout.

“I mean, first time we’ve seen him, it’s pretty nasty between the fastball and the breaking ball -- or two breaking balls,” Blue Jays manager John Schneider said of Miller. “That was a really good arm. Deceptive, kind of quick and impressive stuff.”

Despite the overpowering effort, Miller said he felt “rusty” with all of his pitches, which makes one wonder just how unhittable he might look once he’s feeling at his best.

Miller will get a chance to find that comfort level in this final month, albeit in short stints. Kotsay indicated that the A’s plan is to be cautious with the hurler’s workload over the last few weeks of the season by utilizing him in similar roles to the one he performed Wednesday, with his pitch count unlikely to exceed 50 in a game.

Should Miller get through the rest of 2023 healthy, the plan is to revert him into a full starter entering next season, projecting him as a future ace for this rebuilding A’s club that is now 9-7 over its past 16 games as it aims for a strong finish.

“We’ve talked about managing Mason and really all of our young arms,” Kotsay said. “For Mason, he’ll probably open a time or two over the next 20-plus games. I think it’s a good role for him to be in control of his pitch count and workload to make sure he finishes through this end of the season.”