Very few teams have seen their rotation outlook transform as dramatically as the White Sox in the last year. The group entered last season entirely filled by long-term question marks due to health, performance, and age-related reasons. Now, it is one with multiple slots anchored down and an influx of promising arms on the way. Lucas Giolito had a breakout year. Dylan Cease flashed potential. While Carlos Rodón, Michael Kopech, and Dane Dunning all return from injury, the team added Dallas Keuchel and Gio Gonzalez to bring stability to the starting five. The White Sox rotation figures to be more crowded than it has been in years, and this all goes without mentioning Reynaldo López.
López will certainly be in the rotation to start the 2020 season, as the team has been determined to keep him a starter despite many outside the organization deeming him better suited for the bullpen. After being called up in late 2017, López survived 2018 with surface level stats that appeared promising for a young pitcher, but underlying numbers showed that he was due for regression. With a 3.91 ERA, slightly below average strikeout and walk rates, and a full season without any trips to the IL, it seemed 2018 could serve as a decent launching point for a young starter with growth potential. However, a low BABIP, a 5.22 xFIP as a byproduct of a HR/FB ratio well below league average, and unspectacular movement from all his pitches were factors that indicated regression would come in 2019.
Indeed, regression came. His ERA shot up to 5.38, he gave up the most earned runs in baseball and placed in the bottom of the leader-boards for hits, walks and home runs. His HR/9 jumped to 1.71, all despite maintaining a HR/FB ratio that was still below league average. While other peripheral stats show he may have been unlucky to some degree (high BABIP, SIERA decrease), nothing indicates that López will be deserving of a rotation spot even if he settles around the mean.
Something significant needs to change for him to maintain his starting role, and barring an offseason makeover à la Giolito currently in the works, the easiest path towards improvement is through diminishing his changeup usage. López utilized his changeup about 15% of the time in each of the past two seasons. However, it became notably less effective last season, going from a -0.13 wCH/C in 2018 to -1.21 wCH/C in 2019. While it ranked just behind his curveball as having the worst pitch value, other metrics suggest that his changeup is the source of bigger problems for López.
Hitters' wOBA against his changeup jumped from .320 in 2018 to .408 in 2019, becoming the highest among any pitch in López's arsenal. This is mainly attributable to the pitch being crushed when thrown in the zone in 2019.
He went from having a high SLG against the pitch in the zone to having the highest among qualified pitchers. Distinguishing using attack zones doesn't help his case either, as the changeup still has the highest wOBA in the heart and shadow of the zone compared to the other pitches in his mix.
This makes the pitch difficult to justify using, except solely as a chase pitch when ahead in the count. But with the control issues López has, that strategy comes with the risk of missing in the zone and getting pounded. The downside is especially apparent when comparing the pitch to his slider, which is another pitch that is hit hard in the zone, except it compensates with a very low wOBA in the shadow and outside of the zone.
It is also difficult to justify the current usage of the pitch because of how much damage hitters do when it is put in the air. It is the second most frequent pitch to become a fly ball after it is thrown, and it has the second highest wOBA of pitches with that outcome. While it isn't the worst in these rankings, each needs to be put into perspective.
The pitch most that most frequently became a fly ball was his curveball, but a high IFFB% on the pitch and generally weak contact on fly balls meant the pitch didn't significantly hinder him. Meanwhile, the slider may have had the highest wOBA, but it was the least likely of all his pitches to go airborne, and a large wOBA/xwOBA discrepancy suggests some luck may have been on the hitter's side. So while the pitches above the changeup in these categories have their advantages, the changeup does not.
These results don't appear to be extremely unlucky either. Hitters' exit velocity and launch angle against the changeup each went up a few ticks last year. His BABIP on the pitch is close to league average. The pitch has unspectacular movement, and with high xwOBA numbers across the board for the pitch, the numbers simply appear to show a pitch that batters have figured out how to demolish when they make contact.
The one positive argument that can be made for it is its Whiff%, which increased from 2018 to 2019 and generates the most swings and misses of any pitch inside the strike zone.
But the swing and miss generated out of the zone is comparable to his breaking pitches. For a pitch that is crushed when contact is made in the zone, is it really worth the few swings and misses it brings? Probably not.
Is it even worth keeping as a setup pitch? Not really. Assigning wOBA to each pitch type based on the result that happened on the pitch thrown after it, the highest wOBA belonged to the changeup.
Every pitch type thrown after the changeup had a lower Strike% than it does on average except for... the changeup. Who could have guessed that he'd be better at performing an activity the second time consecutively?
It is unclear what López should substitute his changeup usage with. Would sliders and curveballs adequately fill the void? Would a 2SFB/sinker be worth exploring. It's hard to say; but one thing is for certain. If López is hoping to find his footing in the rotation, his changeup is making that rotation spot a lot shakier for him.
