PEORIA, Ariz. -- The Padres learned the hard way last autumn that starting-pitching depth is a precious resource. You simply cannot have too much of it.

They're living that mantra this spring.

On Sunday morning, the Padres swung a trade for A's left-hander Sean Manaea, sending a pair of prospects to Oakland in return. Manaea, who is in the final year before he hits free agency, bolsters an already deep San Diego rotation, adding a bit of stability. It might also set up the Padres to deal from elsewhere within their cadre of arms before the season begins.

Infield prospect Euribiel Angeles and right-hander Adrian Martinez went to Oakland in the deal. Angeles was ranked as the team's No. 12 prospect, while Martinez checked in at No. 26. The Padres also received Minor League right-hander Aaron Holiday. Here is the deal in full:

Padres get: LHP Sean Manaea, RHP Aaron Holiday**
A's get:** IF Euribiel Angeles, RHP Adrian Martinez

In a classic Spring Training quirk, Manaea was slated to pitch for Oakland against the Padres on Sunday. Instead, Padres manager Bob Melvin woke Manaea up with a phone call and told him to stay put in Mesa, where he would be pitching for the Padres against his former team.

So Manaea headed to the A’s complex, where he packed his locker and said his goodbyes to his Oakland teammates. Then -- amid a higher-than-usual level of heckling from the opposing dugout -- Manaea proceeded to pitch 3 2/3 innings of one-run ball against those former teammates. In the Padres’ 8-2 loss, Manaea struck out four and walked one in his final tune-up for the regular season.

“I came to the field, and it was a little emotional -- actually very emotional,” Manaea said. “But it was nice to have some closure and say goodbye to the guys. And then, obviously, pitching against them was a little crazy. … It was just a whirlwind, for sure.”

Manaea’s start Sunday could line him up to pitch the Padres’ second game of the season in Arizona on Friday. Yu Darvish has already been tabbed as the team’s Opening Day starter, with Joe Musgrove expected to be pushed to the No. 3 spot, allowing him to pitch the home opener.

“At this point, I’m all in with San Diego,” Manaea said. “The goal is, obviously, to win a World Series. Any way I can help.”

For all the depth in the Padres' starting rotation, they were still looking for some reliability. A number of their starting-pitching options came with injury concerns and have been inconsistent over the past few seasons. In Oakland in 2021, Manaea made 32 starts, posting a 3.92 ERA with a 3.66 FIP, 1.23 WHIP and 194 strikeouts.

Manaea spent all six of his big league seasons in Oakland, amassing 50 wins and a 3.86 career ERA. He authored a no-hitter in 2018, and pitched in two postseasons with the A’s. If anyone knows what Manaea might bring to the Padres, it’s Melvin, who will remain the only manager Manaea has played for in the big leagues.

“You see the durability, you see the numbers,” Melvin said. “He’s a terrific teammate. I’ve always called him ‘World’s best teammate.’”

Manaea's presence gives the Padres a starting-pitching mix that seemingly has eight other potential options:

Darvish
Musgrove
Mike Clevinger
Nick Martinez
Blake Snell
Chris Paddack
MacKenzie Gore
Ryan Weathers (who was optioned to Triple-A on Sunday)

Not to mention Adrian Morejon, who is expected to return from Tommy John surgery midseason. Gore, meanwhile, is in the midst of a breakout spring and has team decision-makers seriously mulling whether he's ready for a big league breakthrough.

Of course, that deep group of arms comes with some question marks. Clevinger is working his way back from 2020 Tommy John surgery, and Melvin noted that he’s likely to begin the season on the injured list as he’s dealt with some right knee soreness in his build-up. Snell, meanwhile, has progressed slowly this spring. Paddack's 2021 season ended early due to elbow trouble. Darvish’s hip issues have seemingly resolved, but that much won’t be absolutely certain until his workload increases.

Enter Manaea. It's still possible that, with their newest acquisition on board, the Padres would look to deal from that level of rotation depth. Last week, they explored a trade with the Mets that would've sent Paddack to New York.

But it’s also possible that the Padres simply enter the season with one of the sport’s deepest rotations. There are worse quandaries to have. Remember, just last September, the Padres’ staff was so depleted by injury that waiver pickups Vince Velasquez and Jake Arrieta were forced to make starts in the team’s biggest series of the season in St. Louis.

This year, the Padres have made it quite clear that pitching depth is a priority. They signed Martinez in the offseason. They’ve welcomed Clevinger back into the fold. Gore is on the verge of a breakthrough. Now, Manaea has joined the group, as well.

“It’s something that, I think because of last year, the organization is looking a little bit differently this year,” Melvin said. “[We’re] trying to accumulate as much depth in the rotation as we can.”

And they certainly helped their cause on Sunday.