OAKLAND -- Last September, Joey Estes was a 20-year-old pitcher finishing up his first season at High-A Lansing. Just over a year later, he found himself on a big league mound squaring off against the likes of superstar Julio Rodríguez and the rest of a playoff-contending Mariners squad.

Estes made his Major League debut in Wednesday’s 6-3 A’s loss at the Coliseum, allowing six runs (five earned) on six hits and one walk with two strikeouts across 4 2/3 innings. At 21 years and 347 days old, Estes became Oakland’s youngest starting pitcher to make his debut since Brett Anderson (21 years, 68 days) in 2009.

“For this young man, at 21, to be making a Major League debut, says a lot about the organization’s belief in his abilities,” manager Mark Kotsay said. “The confidence we have in him as well. Through this season, he’s been able to make an impression that he’s capable of coming here and competing at the highest level.

“It’s an exciting day for the organization and for him.”

Reaching the Majors at such a young age is no easy task, particularly for a lesser-hyped amateur player like Estes, who was a 16th-round pick out of Paraclete High School (Lancaster, Calif.) by the Braves in the 2019 MLB Draft.

But since his acquisition from Atlanta as part of the return for Matt Olson in 2022, Estes has been described by Oakland’s scouting department as somewhat of a revelation.

Despite not being invited to big league camp this spring, Estes has ascended through Double-A and Triple-A this season, combining to post a 3.74 ERA in 27 games (23 starts) with 131 strikeouts over 137 innings.

Officially getting his contract selected from the Minors on Wednesday morning, he arrived to the A’s ranked as the club’s No. 13 prospect by MLB Pipeline.

“I was just working towards a goal,” Estes said. “I wasn’t really focused on when they would call me up. I just wanted to pitch and give the best of my ability out there.”

On hand at the Coliseum to take in Estes’ debut was a large contingent of friends and family members, including his parents, Joe and Elise, and two brothers, Nick and Cameron. The group sat about 10 rows behind the A’s dugout and greeted Estes with a standing ovation each time he walked off the mound.

“Everyone you could think of was all here,” Estes said. “It’s awesome to have them here supporting. I heard them all the whole time. I’m just super thankful and grateful for them.”

Estes admittedly found it difficult to shake off the nerves that typically come with a debut. He “couldn’t feel” his legs in the first two innings, yet he fared well in the early goings, limiting Seattle to one earned run through four frames.

Facing Estes a third time through the order in the fifth was when the Mariners really broke through with back-to-back homers by Rodríguez and Cal Raleigh.

Still, the A’s saw plenty to like from the rookie. He pounded the zone with 60 of 90 pitches going for strikes while generating 12 whiffs, including seven on his fastball, which maxed out at 95.4 mph.

“He’s a guy that is not going to beat himself,” Kotsay said. “He pounds the strike zone and changes speeds. It looks like he has a good knowledge of what he wants to do out there. For a first outing, he didn’t give up [many] free bases. …

“I was happy with what I saw. The fastball plays when the changeup is down, and I think he’s got enough of a mix to have success.”

With 10 games left on the schedule, Estes will get at least one more start with the A’s before season’s end. While the final line was not up to par with the standard Estes has set throughout a successful Minor League career, Wednesday was nonetheless an experience he will never forget.

“It was surreal,” Estes said. “I’m still kind of shaken up from it, to be honest. … I’m not really worried about results and the whole thing right now. I’m just happy to be living in this moment.

“I worked very hard to get here, so I’m just living in the moment.”