OAKLAND, Calif. -- On paper, the Oakland Athletics looked like a long shot at best to beat the Seattle Mariners on Thursday night at the Oakland Coliseum.

Seattle started left-hander James Paxton, who hadn't given up a run in 21 innings to start the season, a franchise record.

The A's countered with 32-year-old right-hander Cesar Valdez, who was called up from Triple-A Nashville and made his first major league appearance since 2010 and his third-ever big league start.

Paxton gave up five runs over 4 1/3 innings, Valdez yielded just three runs over four innings, and the A's wound up powering their way to a 9-6 victory.

Trevor Plouffe hit a three-run homer, and Ryon Healy went 3-for-3 with a double and two RBIs, leading the A's to their straight win. But it was Valdez, who started in place of the injured Kendall Graveman, who owned the spotlight with his long-awaited return to the major leagues.

"That's amazing," A's center fielder Rajai Davis said. "Seven years. He gave us a chance to win. He obviously had to settle down and start pitching to the corners, and once he started doing that, he was able to keep them off-balance with his delivery and a lot of trick pitches. It worked."

Paxton blanked the A's in the first and second innings, but his scoreless streak ended at 23 innings when Oakland scored three runs in the third. Paxton allowed nine hits, struck out eight and walked one.

"I was just missing over the middle with my fastball a little bit, and I wasn't landing good curveballs for strikes," Paxton said. "So they just started looking fastball, and I wasn't hitting good spots tonight."

Valdez gave up one run in the first inning and two in the second but blanked the Mariners in the third and fourth. Even so, with Graveman likely to make his next start, Valdez's stint with the A's could be short.

"Whether that chance comes again, the second chance, I want to be able to give the best of me and come out there and do everything I can," Valdez said. "I'm happy that we won, and it's an exciting time for me, but whatever's asked of me next time around, I'm ready for it."

The A's led 6-5 entering the bottom of the seventh inning. After Mariners reliever Dan Altavilla walked Khris Davis and Healy with one out, Plouffe crushed his next pitch over the left field fence for his fourth home run of the season to make it 9-5. Plouffe, who struck out his first three at-bats, hit a 96 mph fastball that was down and inside, a few inches off the plate.

"I've been 0-2, 1-2 a lot lately," Plouffe said of the counts he has faced. "Some of that is because I haven't seen these (pitchers) before and I'm trying to get a feel for what they have, and when you're 0-1, 0-2, it's not a good place to hit. So that at-bat, I didn't want that to happen again, so I geared up for a heater and I got it."

Rajai Davis went 2-for-5 with an RBI and scored two runs for the A's but left the game after the bottom of the eighth inning with a left hamstring injury he sustained running out a ground ball in the sixth. He won't be in the lineup on Friday night and isn't sure if he will miss more time than that.

Oakland's Jed Lowrie had two hits, including a double, and scored once. Ryan Dull (1-1) pitched 1 1/3 scoreless innings in relief for the win.

Taylor Motter went 2-for-4 with a two-run homer, a double and two runs for Seattle.

"Taylor continues to put good swings on it," Mariners manager Scott Servais said. "He's on the fastball, and he's having good at-bats for us."

The Mariners trailed 5-3 through five innings, but Nelson Cruz hit a leadoff double in the sixth off reliever Frankie Montas, and Motter lined a two-run homer to center with one out.

Oakland regained a 6-5 lead in the bottom of the inning when Josh Phegley drilled a leadoff double off Evan Scribner (0-2), moved to third on a sacrifice bunt and scored on Rajai Davis' ground out.

NOTES: C Bruce Maxwell was optioned to Triple-A Nashville by the A's, clearing a roster spot for the arrival of RHP Cesar Valdez. Oakland moved SS Marcus Semien (wrist) from the 10-day disabled list to the 60-day DL to open a spot for Valdez on the 40-man roster.... Mariners 1B Danny Valencia faced the A's for the first time since they traded him to Seattle on Nov. 12 for RHP Paul Blackburn. He went 1-for-3 with a double and a walk. ... Mariners rookie RF Mitch Haniger went 0-for-4 with a walk, ending his 13-game hitting streak. ... Jed Lowrie played his 69th consecutive errorless game as a second baseman, the fourth-longest such streak in Oakland history.