OAKLAND -- When Darell Hernaiz arrived at the Coliseum on Monday, he found that his assigned locker was located directly next to manager Mark Kotsay's office. The A's skipper invited the 22-year-old in for a chat, joking that he had placed Hernaiz's locker nearby so he could keep an eye on him.

Then Kotsay shared the big news: Hernaiz had earned a spot on the A's Opening Day roster.

"I just kind of started smiling, kind of was trying to play it cool," Hernaiz said after Monday's 4-1 loss to the Giants. "I shook his hand, and about five seconds later, I just started crying. … I'm on cloud nine right now, so it's awesome."

That smile was as radiant as ever hours later as Hernaiz recalled calling his mother, Edna Gonzalez, to let her know -- while he was still in the office with Kotsay, "because I didn't think she was going to believe me."

Calls to his father, Juan, and wife, Lauren, followed, with friends and former coaches also receiving the good news.

Taking part in his first big league camp this spring, Hernaiz, Oakland's No. 5 prospect according to MLB Pipeline, has lived up to the hype that he's created through his strong body of work in Oakland's farm system -- and he's not planning on peaking with his first call to The Show.

"There's a lot of work to do," Hernaiz said. "I'm, in my opinion, nowhere near my finished product."

Per the club, Hernaiz (22 years, 238 days old on March 28) will be the youngest player on an A's Opening Day roster since Brett Anderson in 2010 (22 years, 63 days).

"We talked, when he got here, about the things that we felt he needed to improve on and show growth in, both on and off the field," Kotsay said. "And I think he's definitely done a good job of that as a young player, maturing over these last 40 days with this group."

Hernaiz was acquired from the Orioles in exchange for Cole Irvin in January 2023. He began that year with Double-A Midland, then earned a promotion to Triple-A Las Vegas in July. Across the two levels, Hernaiz slashed .321/.386/.456 with 32 doubles, nine homers and 71 RBIs in his first season with the organization.

Primarily a shortstop in his professional career, Hernaiz is also capable of handling the hot corner. Entering Spring Training, Hernaiz's best shot at cracking the Opening Day roster appeared to be winning the starting third-base job -- until the A's signed veteran J.D. Davis to a one-year deal.

Even with a less obvious fit on the roster, Hernaiz forced the issue by hitting .306 with a .333 on-base percentage in 20 exhibition games while also making strides defensively, standing out to coaches and teammates alike.

"I think he's come in and impressed a lot of people, both defensively and then with his approach at the plate," designated hitter/outfielder Brent Rooker said during Spring Training. "The way he handles the bat, his knack for situational hitting and just going out there and putting up a quality at-bat, being a difficult out every time."

Said JP Sears, Monday's starting pitcher: "I wasn't surprised. … He's been a really good guy to get to know. Really good tools. I think he's just going to be a really good addition to our lineup, just the type of hitter he is."

Rather than guaranteeing Hernaiz everyday at-bats with Triple-A Las Vegas, Kotsay is confident that he can find regular opportunities for him to get the reps he needs. For the time being, the plan is for Hernaiz to back up Davis at third base and Nick Allen at shortstop.

"He's got an opportunity here to play short, to play third, to pinch-hit late in games, come off the bench and run, and also come off the bench and be a defensive replacement," Kotsay said. "Darell has done a great job this spring at both positions. There's certain facets, I think, at shortstop that we're working on him with, being a little more aggressive in his play."

When Hernaiz debuts, he'll be the latest addition to an A's youth movement -- notably featuring second baseman Zack Gelof, catcher Tyler Soderstrom and outfielder Lawrence Butler -- that began to make its mark on the big leagues in 2023.

Now that he's cracked the roster, Hernaiz is willing to fulfill any role the club needs.

"In the big leagues, it's all about winning," he said. "And all I want to do is help the team win."