Things were moving right along on Sunday for Justus Sheffield until they all of a sudden weren’t, and a snowball fifth inning cost the Mariners in a 10-2 loss to the Rangers at Globe Life Field.

With a one-run lead and the Nos. 8-9-1 hitters due up, Sheffield was at 50 pitches and cruising entering the fifth. But the left-hander hung a first-pitch slider over the plate for a leadoff homer to Charlie Culberson, then a rare throwing error from J.P. Crawford allowed the next batter, Jose Trevino, to reach, and from there, Texas was off and running.

Isiah Kiner-Falefa singled, Nick Solak doubled, Nate Lowe singled and Adolis García homered before Sheffield recorded his first out of the frame -- a six-run swing. His seven total runs represented a career high for the 24-year-old, who has shown glimpses of his stellar 2020 rookie season at times in '21, but he’s yet to exhibit that poise consistently.

Sheffield didn’t have the command, conviction or confidence in his elite slider on Sunday, which when it’s on is one of the best in the American League. But for a three-pitch starting pitcher, when any of his weapons are off, he finds himself playing catch-up far more significantly than those who have a deeper arsenal.

Entering Sunday, opposing hitters were batting .167 and slugging .389 against Sheffield's slider. But Texas tagged it for a 6-for-12 day, including both home runs.

“Just late in the count, just not really throwing it in the dirt and getting swings and misses like I want to,” Sheffield said. “I left a couple of them up -- got away with some of them -- and paid for some of the ones that I did leave up.”

Sheffield doesn’t possess overpowering velocity or spin on his two-seam fastball, so while it is an effective ground-ball pitch, it generates almost no swings and misses (7.6% whiff rate entering the day). And his changeup remains a work-in-progress. There are times when he’s been able to effectively thwart right-handed hitters on the outside of the strike zone, but opposing hitters were still batting .444 and slugging .500 against it entering Sunday. Sheffield essentially ditched his four-seamer last year because the behavior of the two-seamer was more suitable for his game.

All of that is to say that Sheffield needs that slider at full strength.

“I feel like I got ahead well,” Sheffield said. “I felt like I just didn't make some pitches when I was ahead. I left some pitches up that obviously they took advantage of. I can't do that. Big league hitters, they're gonna hit that pitch, especially left up over the plate.”

At the plate, the Mariners weren’t able to manufacture runs the way they did the first two games of the series, but they strung together 10 hits to finish the series with 31 -- their most in any three-game stretch this season and an encouraging sign after being no-hit on Wednesday by Baltimore's John Means. That no-no represented the culmination of the offense’s collective struggles finally coming to a head, so the bats getting going in Arlington proved to be a positive, despite not being able to generate much against the Rangers’ bullpen.

Mitch Haniger and Kyle Seager accounted for Sunday’s two runs with a pair of RBI singles in the fifth. It was Haniger's first start of the season not hitting leadoff, and Seager moved into the cleanup spot as part of a minor shuffle. Haniger hit second behind Crawford, who went 2-for-5 and has been Seattle’s hottest hitter, reaching base in 25 of his past 28 games. Batting third, Kyle Lewis went 2-for-5, finishing the series 7-for-13. He is hitting .258 with a .749 OPS.

Ty France, who’s in a 2-for-40 funk, had a scheduled day off to piggyback with Monday’s off-day ahead of the Mariners’ upcoming two-game series against the Dodgers.

“We're making some strides offensively,” Mariners manager Scott Servais said. “I'm happy to see a lot of guys using the whole field. We saw a lot of hits go the opposite field in this series. It's something we needed to get back to doing coming into the series. So we're making some adjustments there, and it's paying off. There were a lot of hits in the opposite field. We certainly saw what Texas did with it this series -- I thought they had a really good approach against us.”