SEATTLE -- In a season that was expected to feature more downs than ups, the Mariners anticipated that they would likely see more highs than lows from their most significant offseason acquisition.

But Yusei Kikuchi appeared to still be seeking answers in what’s been a challenging rookie season after Sunday’s 9-3 loss to the Angels, his third in five starts against the division rival this season, as the two clubs concluded their season series with L.A. taking 10 of 19.

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The Mariners lost their sixth straight series and for the 15th time in their past 18 games dating back to June 27. They are 27-60 since starting a MLB-best 13-2, for a win percentage of .310 that ranks ahead of only Detroit’s .268 in that stretch.

Kikuchi was tagged for seven earned runs over five innings, including homers from Albert Pujols and Brian Goodwin, ballooning his ERA against the Halos to 11.32 over 20 2/3 innings. Shohei Ohtani went 1-for-3 against Kikuchi with a double to finish the season 3-for-7 against his fellow Japanese countryman. Ohtani also homered against Kikuchi in their first meeting on June 8.

But the opponent on Sunday doesn’t necessarily reflect Kikuchi’s biggest obstacle, which has been a surplus of balls in play. On Sunday, he had 21 such cases, two shy of his season high, but he did so on just 74 pitches for a rate of 28.4 percent, which is by far a season high. That ranks in the 99.5th percentile among 2,000 starters this season in which the pitcher threw at least 70 pitches.

For the season, opposing hitters are putting the ball in play on 19.8 percent of Kikuchi’s total pitches, and they’re batting .340 and slugging .596 when doing so. For context, the MLB average for starting pitchers allowing balls in play is 17.3 percent.

“Even though today wasn't what I wanted, I felt like I was throwing pretty well with my fastball,” Kikuchi said. “It had some good run on it. Moving forward, I do have to work more with my fastball and curveball mix.”

“The fastball command at times has been better than it is at other times,” Mariners manager Scott Servais said. “He's still going through a learning process. We still like the stuff and where it's at. Fortunately, we won't have to face the Angels again this year.”

The biggest culprit on Sunday was Kikuchi’s four-seam fastball, which the Angels put in play 12 times and converted into seven hits, including both homers, two singles, two doubles and a triple. Entering Sunday, Kikuchi was allowing a .324 batting average and a .560 slugging percentage against his four-seamer. Among 82 starters who’ve thrown at least 500 four-seamers, those figures rank eighth- and 11th-worst, respectively.

Cognizant of the urgency to be more deceptive, Kikuchi has slowly been installing a changeup into his repertoire, but it’s admittedly still a work in progress. He threw it seven times on Sunday, and induced two groundouts, with no hits.

“One through nine, they have a really good balance in their lineup,” Kikuchi said through an interpreter. “Everyone can make contact. Everyone can drive the ball. That's what I've noticed. … This is a really high level, so I've been trying to adjust to that. I've been working on my changeup and throwing it more in games. But it's a game of adjustments.”

The Mariners anticipated that Kikuchi would need to adjust to a more rigorous Major League workload after spending eight seasons in Nippon Professional Baseball, where he typically pitched just once per week, as well as the grip on the ball, which is more slippery and has lower stitching in MLB opposed to NPB.

To help curb his workload, Seattle used Kikuchi as an opener in an April 26 win against the Rangers, after which he went on a stretch of four starts with a 2.03 ERA before stumbling with back-to-back 3 1/3-inning outings that prompted the club to skip his turn through the rotation in early June.

But there are still 10 weeks left in the regular season that necessitate innings to eat -- particularly for a Seattle club that has already used a MLB-high 36 pitchers this season. Servais said Kikuchi will make his next scheduled start on Friday against the Tigers.

The Mariners have only won one of Kikuchi’s past 10 starts, and he dropped to 1-6 with a 7.85 ERA in those outings. To be sure, Seattle’s offense couldn’t muster a run of support for Kikuchi, and it didn’t plate any runs until the seventh when trailing 8-0.

Of course, it’s also been a mentally and emotionally taxing 2019 for Kikuchi away from the field. He migrated to the United States after signing a three-year, $43 million contract with a fourth-year club option on Jan. 2, lost his father in April and became one earlier this month.

“There's a lot going on, but he continues to work his tail off,” Servais said. “We fully expect him to learn, and he'll bounce back and get it trending in the right direction as we head down the stretch.”