SEATTLE -- Alec Marsh is not one to shy away from tinkering with his stuff in the middle of the season in search of better results. A few weeks ago, he changed the grip of his slider to elicit better movement on the pitch.

On Sunday, he flashed a new sinker in an effort to cover more of the plate and help his four-seam fastball.

Marsh struck out nine and allowed three runs in 5 2/3 innings in the Royals’ 3-2 loss to the red-hot Mariners at T-Mobile Park on Sunday, ending the series with a sweep. Home runs continued to hurt Marsh, with all three of Seattle’s runs coming on two homers, but the rookie still showed progress, starting a game for the first time since Aug. 5 after three bulk relief appearances.

“Whatever role they give me, it is what it is, I just want to get the ball,” Marsh said. “I would love to start, continue to start, but I’ve got to continue to earn that right. Keep the walks down, keep the pitch count down, go deeper in the games. …

“The home runs are still an issue. But we’re working on some stuff that’s seeing some really positive outcomes.”

Nelson Velázquez’s two-run home run to right-center field brought the Royals within a run in the eighth, but they are heading home after a 2-7 road trip -- and all but two of those games were decided by two runs or fewer. With their 91st loss of the season, the Royals dropped to 50 games below .500 for the first time since 2018.

“Super frustrating series,” manager Matt Quatraro said. “Really a frustrating road trip. We played better than what the results ended up, except for [Saturday’s 15-2 loss]. Don’t take a whole lot out of it -- they’re still losses. Close losses or not, they’re still losses.”

Marsh got the chance to start Sunday because the Royals didn’t feel like they had the right opener candidate, as they typically look for the right matchups and a power arm. The 25-year-old took what he’s been working on in bullpen sessions to the game: Adding a sinker to his repertoire. He only threw it periodically but got a couple ground balls on it, including to induce a Ty France double play in the fourth inning.

Marsh hasn’t thrown a sinker since he was drafted in 2019, but he views it as a good option to pair with his four-seamer, which is focused at the top of the zone. He has success there -- five of his nine strikeouts Sunday came on swings on an elevated fastball -- but also misses up or generates a lot of foul balls, leading to longer counts and driving Marsh’s pitch count up.

“If there’s something I can throw down in the middle of the plate and it’s going to produce more ground-ball contact and let my defense work, it’s only going to help me, and it’s going to allow me to go deeper,” Marsh said. “It gives me a different focus. If I have a two-seam, I can just throw this in the middle of the plate and let it run in the hands. So I don’t have to be so careful.”

Marsh had some trouble with his slider on Sunday because the new grip he’s using causes his fingernail to crack, and he’s learning how to pitch with those issues. But he still generated five whiffs on 13 swings on sliders (38 percent), including a knee-buckling one to Josh Rojas in the second inning.

But both of the home runs Marsh allowed were on sliders he mislocated in the zone. Marsh has allowed 14 home runs in 49 1/3 innings to begin his career and has allowed at least one home run in each of his first seven career starts. The one that hurt the most was Julio Rodríguez’s two-out, two-run blast in the fifth after Marsh walked Rojas, the No. 9 hitter.

Marsh and the bullpen still kept the Royals in the game despite the offense’s struggles against Mariners starter Luis Castillo, who dominated with seven scoreless innings.

“I’ve faced [Castillo] a couple times now, and that was as good as I’ve seen him,” said second baseman Michael Massey, whose left-on-left single in the eighth set up Velázquez’s home run. “Today, that fastball up top was electric for him. … It’s been a really frustrating trip. Go back, try to regroup. We’ve got a month left. So we’ve got to find a way to fight and finish strong.”