Jed Lowrie ambushed a 3-0 fastball from Hernandez in the first inning for a two-run homer after a blown call on a balk left a runner on base.
"I was trying to go sinker, down and away," Hernandez said, "but it didn't sink a lot. I didn't even know he was going to swing because he's patient. But he did swing. That cost us the game right there."
Those two runs were enough. Manaea (2-2) limited the Mariners to one run and two hits in seven innings before Ryan Buchter and Blake Treinen closed out the victory. Manaea lowered his ERA to 1.63.
"Boy, I don't know where we'd be without him at this point," Athletics manager Bob Melvin said. "You know, he saves the bullpen. He's pitched great and won games for us."
The Mariners got their only run when Taylor Motter turned on a one-out fastball for a homer in the fifth inning.
"I want to hunt heaters," Motter admitted. "I'm a fastball hitter. I want to hit the fastball, but he mixes his pitches well, and he mixes speeds well. He's really coming from a tougher angle for a lefty to see."
Manaea muffled a Mariners attack that produced 29 runs in winning its four previous games.
"Give Manaea credit," manager Scott Servais said. "Offensively, we've been going so good that you didn't expect to see that type of game out there today. But that's baseball. That's the beauty of it."
It was a frustrating loss for Hernandez (2-2) against an opponent he typically dominates -- 25-9 with a 2.60 ERA in 46 previous starts. Particularly frustrating was a mistaken call on a balk by umpire Carlos Torres in the first inning.
Hernandez appeared to pick off Marcus Semien before Torres called a balk. When the Mariners protested, the umpiring crew, after a conference, reversed the call, ruled "no play," and sent Semien back to first.
"It wasn't a balk," Hernandez said. "I stepped off the rubber. I don't know what happened there. It was a weird play. They put back the runner at first. I don't know why. It's frustrating. It should have been an out."
Lowrie followed with a homer.
Hernandez allowed nothing more in pitching into the seventh inning.
SOUND SMART Robinson Cano has become the walking man, which is a big reason for his .537 on-base percentage. His walk in the first inning was his 12th of the season and marked the fifth straight game in which he's worked at least one free pass. Cano has also reached base at least once in all 13 games, which is the longest such streak to start a season in his career.
MITEL REPLAY OF THE DAY Dee Gordon's bid to extend his hitting streak to 13 games ended in the eighth inning when Athletics third baseman Matt Chapman made a stellar play on what appeared to be a perfect bunt. The Mariners challenged Torres' call, but a replay review confirmed the decision. Instead of a single, Gordon was credited with a sacrifice.
UP NEXT The Mariners get their first look at the reigning world champs Monday at 7:10 p.m. PT, when Houston arrives for a four-game stay at Safeco Field. It's been a challenging matchup for the Mariners in recent years: 20-37 since the start of 2015. The opener offers a matchup of tough lefties: Seattle's James Paxton vs. the Astros' Dallas Keuchel.
Bob Dutton is a contributor to MLB.com based in Seattle.