While Kyle Lewis’ breakout has been the biggest story for the Mariners this season, the development of fellow rookie Justus Sheffield stands as a strong contender.

The 24-year-old lefty wrapped up his first full season in the Majors with five innings of one-run ball in Seattle’s 5-1, eight-inning win over the A’s in the first game of Saturday’s doubleheader in Oakland, giving him a final record of 4-3 with a 3.58 ERA in 10 starts.

• Box score

“I know we talk about how Kyle Lewis is probably going to end up winning Rookie of the Year,” Mariners manager Scott Servais said. “This guy is right there with him, with what he’s been able to do and how consistent he’s been. He’s one of the best young pitching prospects in this game, and he’s showing it at the Major League level.

“It’s awesome for his future and awesome for what we’re trying to build here going forward. You just need those guys. You can’t have enough starting pitching that is reliable. And he’s getting better.”

J.P. Crawford and Ty France had RBI singles in the top of the eighth off A’s standout closer Liam Hendriks, and Kyle Seager followed with a two-run homer off Lou Trivino, as the Mariners won under this year’s new rules, where doubleheader games are just seven innings and extra-innings start with an automatic runner on second.

Sheffield wasn’t as sharp in his finale, as he gave up five hits with two walks and one strikeout, but he turned a 1-1 tie over to Seattle’s bullpen. In his final eight outings, Sheffield went 4-1 with a 2.64 ERA, with 41 strikeouts, 14 walks and six quality starts, and he said there’s still more in the tank.

“I feel good, but there’s always work to be done,” Sheffield said. “I feel I can be even better than what I showcased this year. I’m going to put in the hard work this offseason and come back even stronger.”

Crawford went 3-for-4 with two RBIs and a stolen base, and he continued his strong defensive play at shortstop. Rookie outfielder Braden Bishop, recalled from the alternate training site on Wednesday after Jake Fraley strained his right quad, was 2-for-2 with a double and run scored.

Kendall Graveman earned the win with two scoreless innings in relief, his first victory since May 11, 2018, when he was with the A’s. Graveman battled back from Tommy John surgery to open this season in Seattle’s rotation, but he was then sidelined by a benign tumor in his neck before returning in a bullpen role that could set up his future.

Graveman has a $3.5 million club option for 2021, but the 29-year-old said he’ll need to talk to Servais, pitching coach Pete Woodworth and Mariners general manager Jerry Dipoto when the season ends to figure out where things go now.

“I’m kind of at the mercy of how my body is,” Graveman said. “One thing I do know is I feel great right now. I am not having any issues like I was for the couple starts that I had, so that’s a blessing. Honestly, I’m just thankful that I feel good.

“So going into this offseason, I don’t know if the tumor leaves my body or if I’m healed from it. But what I do know right now is I’m able to do this and feel great. So that’s the mindset going in, if something changes with my body we’ll re-evaluate. But I feel like I can be a reliever and be a pretty good one.”