SEATTLE -- In a season to be judged largely by the development of their younger nucleus of core players, the Mariners can’t help but be concerned by the recent struggles of Japanese left-hander Yusei Kikuchi.

The 27-year-old rookie was the Mariners’ premier free-agent addition last winter, signed as a promising piece for the rotation going forward. But Kikuchi took his fourth straight loss on Tuesday in a 9-0 setback to the Royals, allowing six runs on nine hits in five innings.

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Though Kikuchi had pitched better in his previous appearance in Minnesota, he’s now 0-4 with a 10.35 ERA over his last five starts, putting him at 3-5 with 5.15 ERA in 16 outings this season.

Manager Scott Servais acknowledged Kikuchi didn’t have his best fastball or command from the start and needs to learn how to adjust on those kind of nights.

“We knew this was going to be a process, bringing him over here,” Servais said. “But certain nights where you don’t have your crispest stuff or your best fastball, those are the nights you really find what kind of pitcher you are.

“He’s had a few of those where he may be right out of the chute hasn’t been throwing the 94-mph heater and nasty slider. But you still have to find a way to keep your team in the game, and he’s struggled to do that. And that’s on us, the pitching coaches and myself, to help him along in those areas. Certainly we’re working a lot on it.”

Kikuchi noted his fastball was only around 91 mph most of the night, a few ticks below his normal velocity, though he said he’s fine.

“Just in general, my velocity wasn’t really there, so I didn’t really have that pitch to jam hitters or kind of push hitters,” Kikuchi said through interpreter Jason Novak. “Times when I had to ramp it up, I’d throw it and look up and it was only 90-91. Compared to when I’m going good, it’s different.”

The Mariners saw Kikuchi throwing in the mid-90s earlier this year, so that certainly will be something to keep an eye on as well.

“It fluctuates a little, and I think that comes off the timing in his delivery,” Servais said. “He’s got a lot of moving parts in there. You’ll even see it fluctuate between innings, from maybe the second inning to the fourth inning, all the sudden he finds it. It’s a timing issue. It’s not health-related or anything like that. It’s more of the timing in his delivery as much as anything.”

The Mariners dropped their second straight to the Royals, who clinched their first winning road series this season while improving to 25-48. Seattle falls to 31-46, including an 18-44 mark since sweeping Kansas City in a four-game set at Kauffman Stadium in mid-April.

Having traded veteran sluggers Edwin Encarnacion and Jay Bruce and with Mitch Haniger on the 10-day injured list, the Mariners’ lineup is missing some of its earlier thump, and that was evident as Homer Bailey (6-6, 4.82 ERA) threw 7 2/3 scoreless innings while allowing just five hits in his longest outing of the season.

While the Royals slugged three homers -- including two from leadoff man Whit Merrifield -- the Mariners were held without a home run for just the 14th time in 77 games this season. They’re 0-14 in those homerless games.

It was just the fourth time the Mariners have been shut out this season and first time since May 1.