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Adding Justus Sheffield to the Seattle Mariners rotation may legitimately improve the club’s odds of winning the 2019 World Series. But the southpaw indeed has room for refinement and that path starts Sunday in Sacramento.

Sheffield, 23 in May, primarily works with a fastball, slider and changeup and displayed all three in working fashion this spring.  There’s a curveball in the arsenal but it’s well below average and he may very well have shelved the pitch for good at this stage.

But the three he does use are plenty good.

Sheffield’s first start of the season isn’t likely to be one that ends in 100 pitches and depending how the club wants to use him in the majors this year — at some point, we’re going to see him with Seattle — Start 1 might not give us much of a hint, either.

Regardless, here’s what I expect and hope to see from Sheffield.

Use the fastball to both sides of the plate and occasionally at the top of the zone — or above it — to keep hitters from getting comfortable and move their eyes vertically. To right-handed batters, Sheffield’s best chance to pitch inside is to be effective away and come inside strategically with the fastball and to bury the slider at the back foot.

The left-hander is also adept at backdooring the slider to righties, which could be an important weapon for him in the long run.

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If he’s efficient, he’ll see the lineup a third time, which is about when there’s more stress to get ahead with someone other than the fastball. In Peoria, he showed he could throw the changeup for strikes as well as get some swings and misses. Doing so may allow him to get deep into games.

Sheffield’s slider flashes plus and versus left-handed batters he’s basically a situational reliever, allowing LHBs a .176 average last season in Triple-A, thanks to some deception and the sweeping action on the breaking ball.

The fastball has life up to 95 but not a lot of plane or sink, which suggests commanding it is Job 1, which isn’t unique to Sheffield, but it may be the difference between No. 4 starter Sheffield and a version that’s closer to a No. 2 starter.

He will miss bats with all three pitches, the slider being an option no matter who’s at the plate. Sheffield’s tough when he gets to 0-2, 1-2, 2-2, but getting there has been his lone challenge as a pro.

I expect a heavy dose of changeups versus a right-handed heavy River Cats lineup, but we’re going to see the slider when he gets to two strikes.

When Sheffield uses his lower half well, the action on all of his pitches ticks up and his command and control sit in the average range.

He’s likely slated for somewhere in the 75-85 pitch range Sunday.

I’m curious what the overall plan is for Sheffield since he’s major-league ready despite some needed polish, but not for a 180-plus inning workload, which made starting him in Tacoma not only expected but understood.

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There’s probably no chance the Mariners get stupid with service time, though the workload concerns probably help them avoid Sheffield hitting arbitration status until after the 2022 season.

I’d imagine somewhere in the 150-160 innings range is where the Mariners want him — his career high of 127.2 innings occurred four years ago, his first full year in pro ball and he ended 2018 with 118.2 combined innings split between Triple-A and the majors.

So how many starts is Sheffield likely to make in Seattle?

I’d guess at least eight and as many as 16, but if I had to wager I’d say 14, depending on progress and health. That gets us to the end of June. I’d be surprised if we didn’t see him called up by the time the post-break schedule begins.

How will he be used in Tacoma?

Likely in the rotation regularly, on a stingy pitch count that slowly is ramped up, and susceptible to pushbacks, short-scripted starts and skips.

How will he be used in Seattle?

Probably as a starter, but I wonder if the club considers using him as the piggyback to Yusei Kukuchi’s short-scripted starts. Someone’s got to cover those innings, it might as well be Sheffield and Erik Swanson as much as anyone else.