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Keynan Middleton is exactly the Mariners’ type, sort of

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The hard-throwing reliever fits a familiar organizational pattern

MLB: Houston Astros at Los Angeles Angels Pool Photo-USA TODAY Sports

In a continued effort to overhaul the bullpen, the Mariners are presumably nearing the end of the process after acquiring Rafael Montero and Keynan Middleton in consecutive days. Our own Kate Preusser wrote both of them up, but I want to take things a step further and explain why Middleton and the Mariners are such a strong match. He’s exactly the Mariners’ type.

I mean, first, he was cheap! He was signed for less than $1M, and we all know that Jerry Dipoto’s M.O. is acquiring young pitchers for very little, especially from teams that might have done messed up their development. Given that Middleton is just 27 years old, Dipoto appears to have done that! The kicker? He might be quite good, too.

Before I get to Middleton, let’s discuss a few former Mariners pitchers that serve as precedents for what the Mariners could be doing with Middleton. First, Connor Sadzeck, by pitch usage:

Sadzeck joined the Mariners in 2019 and immediately faded his curveball and fastball in favor of his slider. The early results looked promising, as Sadzeck nearly doubled the strikeout percentage he had with the Rangers, but ultimately had season-ending Tommy John surgery and hasn’t pitched since.

Then there was perhaps the most notable example, Austin Adams:

Adams joined the Mariners in 2019 and, once again, dropped his fastball usage in favor of his slider. This change turned him into a borderline elite reliever, as well as the best Austin Adams in MLB.

Perhaps my favorite, though, is Taylor Williams:

Sadly, Williams only pitched 13.2 innings for the Mariners before being traded to the Padres. Before that happened, though, he showcased a similar package to Adams, in that he had an underwhelming fastball paired with a plus slider. Due in part to their fastball-slider combinations, I wrote in August that Williams isn’t not Adams. Speaking of players not being things, they are now both Padres, and not Mariners!

You know where I’m going with this. Or maybe you don’t! My editor-in-chief told me you don’t. Either way, here’s Middleton’s fastball:

97 mph with ride! He misses his spot, but he misses up and in.

Here’s his slider:

His slider has 12-6 movement and is thrown in the upper-80s, which is awfully hard. It doesn’t have much snap to it, though.

As a bonus, he has a changeup too. Pay attention to the catcher’s nonverbal cues to his pitcher:

He taps his glove on the ground pre-pitch, essentially saying, “Bury this.” Middleton doesn’t disappoint, throwing it well below the plate, and the catcher gestures with a point of his glove, emphasizing that Middleton threw what he was looking for.

As I see it, the Middleton is probably headed down the same path as the relievers listed above that had joined the Mariners: more sliders, fewer fastballs. Even though Middleton’s numbers haven’t exactly been striking during his young career — he has a 4.35 FIP over 95.2 innings — he might have the chance to thrive in Seattle.

For Middleton, one issue is that he has historically not had trouble getting to two-strike counts. The problem has been actually putting hitters away. The issue isn’t that hard to tease out. It’s his slider. He should be using his slider as his putaway pitch, not his fastball, as the former is a much better strike-getting pitch.

A table, highlighting slider usage by count:

Adams, Middleton, Williams’ slider usage, by count

Adams Middleton Williams
Adams Middleton Williams
Batter ahead 47.4% 20.4% 47.1%
Pitcher ahead 78.4% 36.7% 55.4%
Two strikes 78.6% 35.5% 71.6%
(Adams’ numbers from 2019; Williams’ from 2020)

Adams is the king of pitch mix optimization. His slider is so good that he can use it whenever he wants when he’s ahead, and hitters will swing. While Williams doesn’t use it as liberally when he’s ahead, he knows to move to it when he gets to two strikes. For Middleton, though, he shies away from his slider in all counts — at least relative to Adams and Williams. That isn’t necessary a slight towards him, it just means that he can probably tweak his pitch mix to better reflect what he should actually be throwing.

That’s not to say that Middleton is behind these other relievers in all facets. His fastball is a lot better than that of Adams and Williams. Obviously, this allows him to throw a fastball 60% of the time that’s netted him an 11.0% swinging-strike percentage — but he doesn’t need to! His career 27.9% fastball CSW (called strikes plus whiffs) is okay, but if he wants to throw more strikes (and thus end more at-bats in strikeouts), he’ll need to lean on his slider more. In doing so, he’d likely give his fastball CSW a boost, and he’s still got a changeup that he does a good job of throwing strikes with, too.

Now, he doesn’t come without his faults. Middleton has dealt with a home run problem against righties throughout his career, and I suspect that’s because the only two pitches he uses against them are pitches he throws in the zone a lot. Generally speaking, the way to remedy that would be to throw one of those pitches outside of the zone more. The problem is that Middleton’s slider doesn’t get many chases — his career chase percentage is 29.8% — which is likely a side effect of his slider being a hard pitch that lacks drop.

I have no idea if the Mariners are going to try and mess with his slider, and this is more theoretical in nature, but there is some evidence that suggests Middleton’s slider plays better when he takes some off of it, although I’ll admit it’s rather inconclusive. In any case, here’s how his swinging-strike numbers look:

Slider, 87 mph or less: 19.0 SwStr%

Slider, 88 mph or more: 19.7 SwStr%

It looks like my hypothesis may be incorrect! Maybe not though. Here’s Middleton’s slider CSW by pitch speed:

Slider, 87 mph or less: 34.5% CSW

Slider, 88 mph or more: 29.3% CSW

Now, there could be several reasons for this. Given that he only has one full season in four years with the rest being partial seasons, perhaps the data is just messy. It could also be that he takes some off of his slider to flip in as a get-me-over pitch. Or perhaps it’s legitimate! Maybe his slider is a better pitch when it comes in slower. For now, we’re forced to speculate, and one angle is a whole new slider could do a lot for Middleton.

Our own John Trupin notes that his slider alternated between shapes and velocities in the minor leagues, sometimes looking like a curveball, and sometimes looking like a slider. If Middleton goes into 2021 with his harder slider, he’s going to be limited, since hitters don’t chase it out of the zone much. Whatever the change is, he needs more separation from his fastball. I think that’s more likely to come in the form of more drop, but a more sweepy pitch wouldn’t hurt him either. Perhaps that’s the Mariners’ entire angle after all.

There’s a strong chance that he’s still a serviceable reliever either way. If the Mariners do nothing, Middleton is probably still a solid pitcher, and he doesn’t figure to factor in as a closer much anyhow. If they tweak his pitch mix or change his slider, there stands the chance that he’s one of the Mariners’ best relievers, and he could pitch himself into some high-leverage innings.

For now, though, we wait. Middleton is healthier than he’s been in a long while, and his fastball velocity reflects that. He may not have the slider that Adams or Williams do, so his slider percentage likely won’t be eclipsing 50%, but he has the opportunity to take a step forward by merely throwing his slider more when he’s ahead. And if the team goes in and fine-tunes his slider, it feels like he’ll be awfully special. In the end, the Mariners may have happened upon a reliever who can close games for under a million dollars.