MLB | Using Data to Diagnose Seung Hwan Oh

Korea’s ‘Final Boss’ was a much easier task for MLB hitters to conquer in 2017

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A much celebrated star on baseball’s international scene, closer Seung Hwan Oh, sent waves through Major League Baseball after signing with the St Louis Cardinals in 2016. The Korean national forced his way into the closers role in that 2016 season, eventually earning 19 saves to go along with a 1.92 ERA and an excellent 27 K-BB%. 2017 proved a whole other story for Oh however, as the right-hander struggled to a 4.10 ERA and a meager 14.8 K-BB%, losing the closer role at times.

While short of a disaster, Oh’s 2017 season was a marked step in the wrong direction – especially as a 35 year old reliever entering his contract year. Nonetheless, Oh apparently showed enough to ink a 1 yr/2.75M deal with the Texas Rangers this off-season. In 2016, Oh was one of the better relievers in baseball. In 2017, the former Cardinal was barely replacement level. The question then becomes – what went wrong for Oh in 2017 and what can the Rangers expect from him in 2018?

Troubles on the Surface

Before we dive in too much deeper, let’s first establish what it is that went from for Oh in 2017. For the sake of brevity let me list them for you:

  • A 12.4% drop in strikeout rate
  • An 11.3% drop in ground ball rate
  • Noticeable struggles against left-handed hitters

To expand on that last point, where Oh had excelled against left-handed hitters in 2016, running a .198 wOBA against opposite handed hitters, that number jumped to an untenable .407 mark in 2017. Underneath that, Oh experienced a 10% drop in strikeout rate as well as a remarkable 2.82 HR/9 against left-handed hitters in 2017.

So now that we understand what went wrong for Oh in 2017, let’s move on to the why question.

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Perhaps the most obvious place to start when looking at performance decline in pitchers – especially those in their mid-30s – is velocity decline. While Oh’s average fastball did drop by just under 1 mph in 2017, a 93.3 mph fastball probably isn’t going to be the root of the issue and indeed, the fastball was Oh’s best pitch (by Pitch Values) in 2017. Instead, it was Oh’s off-speed pitches that failed to to pull their weight last season with his slider, change-up and curveball all experiencing precipitous declines.

Season wFA/100 wCH/100 wSL/100 wCU/100
2016 1.73 0.68 2.24 0.34
2017 1.29 -6.98 -0.45 -10.59

Seung Hwan Oh Pitch Values, Source: Fangraphs

As you can see, Oh’s fastball held relatively stable from 2016/17, while each of the off-speed pitches struggled greatly. The curveball obviously had the sharpest decline but I’ll put that pitch on the back burner as he has thrown the pitch less than 3% of the time in his career.

A Little Deeper

So with velocity proving largely a non-factor, let’s turn our eye towards Oh’s command. While the fastball has held relatively steady in terms of value over his MLB tenure, as I’ve spoken about at some length recently, we have to understand how pitches play off each other. Here are the two fastball heat maps with 2016 on the left and 2017 on the right.

Source: Baseball Savant

As you can see, the nucleus of the fastball heat map is higher in 2017 and separates out to the edges a little more. As an above average spin rate guy in 2016*, trying to locate higher the zone makes some sense and likely was a point of focus between seasons. Let me leave that there for a second while we take a deeper look at what’s going on. To do that let’s isolate Oh’s problems against left-handed hitters.

Trouble with Lefties

As we established earlier, Oh struggled against left-handed pitching in 2017 thanks to regression in his strike out and home run rates. Thanks to Baseball Prospectus’ Pitch Sequencing data, we know that the fastball-change up sequence was Oh’s primary weapon against left-handers in both 2016 and 17 so let’s look there:

Source: Baseball Savant

Again on the left is Oh’s change up location in 2016 and 2017 is on the right. As you can see Oh’s change up was located significantly higher in the zone in 2017. It shouldn’t come as a surprise that when it comes to generating swings and misses with secondary pitches, being able to locate them at or below the bottom of the zone is crucial and for no other pitch type is this more true than the change-up. After locating his change-up consistently below the zone in 2016, Oh found his change-up sitting more often around the mid-thigh area in 2017. As a result, Oh experienced a 7% decrease in his swing strike rate on the pitch and a 20% drop in ground ball rate. Oh’s HR/FB rate on the pitch also sky rocketed to a ridiculous 66.7%.

So Oh’s go to weapon against left-handed hitters, one which was dominant in 2016, proved entirely ineffective in 2017. Consistently missing above the knees with the change-up meant that rather than slipping underneath the barrel of hitters, Oh found the middle of the barrel resulting in a noticeable increase in line drives and home runs and of course a drop in swings and misses.

From Final Boss To Barrel Finder

With that then, let’s take a look at Oh’s work against right-handed hitters. Again thanks to Baseball Prospectus, we know that Oh employs the fastball/slider sequence almost exclusively against same handed hitters. Of course we’ve already looked at the fastball so let’s take a look at Oh’s slider:

Source: Baseball Savant

As you can see, Oh experienced similar troubles with the slider as he did with the change-up in 2017. As the chart above shows, the nucleus of Oh’s sliders slipped into the strike zone more often than is desired and as a result the pitch became far more manageable for hitters in 2017. As a result, Oh’s swinging strike rate on the slider dropped by some 12% and the ground rate dropped a remarkable 25%.

The Final Tunnel

So then let’s start putting all of this together. As anyone who has followed the site recently will know, we have been doing a lot of work with the new pitch tunneling data available at Baseball Prospectus. Given that we are talking about some specific pitch sequences, let’s go back to that well.

2016 2017
Release 4.44 4.05
PreMax 1.41 1.3
PlatePreRatio 14.3 15

The table above shows some relevant tunneling metrics for Oh’s fastball/slider sequence over the last two seasons. The first two metrics (‘Release’ and ‘PreMax’) describe how similar the two pitches are in the early part of pitch flight (smaller is better), while the final metric (PlatePreRatio) shows the difference between the perceived distance prior to the tunnel point and the true distance at the plate for the second pitch in a pairing (larger is better). As you can see, Oh actually did a better job of getting the pitch ‘into the tunnel’ in 2016 than 2017, however, the higher fastball location Oh utilized in 2017 means that the two pitches showed greater separation last season. All this is to say that pitch tunneling had little consequence on Oh’s slider in 2017.

2016 2017
Release 2.42 3.5
PreMax 1.71 1.47
PlatePreRatio 13.1 12.1

Instead, where we can see the effect of tunneling take effect is in Oh’s fastball/change-up sequence. As you can see, Oh’s release point on the change-up showed an extra inch of separation relative to his fastball in 2017 – giving hitters an early cue that a change-up maybe coming. Skipping down to the PlatePreRatio you can see an inch less separation between the fastball and change-up in 2017 than 2016, fueled by that higher location on the change-up in 2017 – reducing the deception on the pitch.

Tying it all together

When breaking down player performance like this I find it important to be able to build an evidence-based narrative to tie all these individual data points together – after all there’s no sense in engaging in such an exercise unless we can understand why the things are occurring or pave a path for improvement. As we saw in Oh’s fastball heat map, Oh was locating higher in the zone in 2017. As a pitcher who exhibited an above average spin rate on the pitch in 2016, trying to work up in the zone in 2017 and induce swinging strikes and weak fly balls makes a good deal of sense. However, in making that adjustment, the earlier, higher release point required to do so, appears to have leaked into his secondary offerings, forcing them higher and closer to the strike zone. As a result, Oh’s slider and change-up – pitches which had proven successful in 2016 – failed to induce swinging strikes and weak contact, instead resulting in much higher line drive and home run rates.

The good news going forward then is that – holding all else constant – simply changing the intent on these offerings to get them back to consistently hitting below the zone should see a return to more favorable results for Oh. On top of that, some natural regression should be expected for Oh’s home run rate

Of course as a 35 year old pitcher, ‘holding all else constant’ remains a dangerous assumption. Oh has already lost a tick on his fastball velocity and as result, some 100 RPM on the pitches spin rate and at 2262 RPM is now exactly average. Age related decline is always a real risk for pitchers like this but if the stuff can hold up and Oh is able to consistently locate his secondary offerings below the zone, Oh might prove one of the biggest bargains of the off-season.

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