Pitchers are fickle beings. Relief pitchers are really fickle beings. Edwin Diaz, for example, burst onto the scene in 2016. Jeff Sullivan detailed how he generated comical whiffs with both a 98 mile-an-hour fastball and a fwippy, drops-off-the-table slider. He also worked in the zone while doing it, which is pretty much the best combo you could ask for from a pitcher.
But in 2017, Diaz essentially laid an egg. His Ks were down. His walks were up. He couldn’t stay in the zone nearly as much, so batters swung less. When they did bite, they hit him much harder than in 2016. His manager talked about how his mechanics had become wonky. He went from being the game’s 13th best reliever to being its 54th.
What’s curious about those wonky mechanics is that they appear to have only burdened his fastball. Not his slider.
Diaz throws his fastball nearly 70% of the time. More than just impacting what was in the zone and what was out of it in 2017, though, his wild tendencies with the heat also appeared to influence his pitches on the edges of the zone. Hitters were more willing to take their chances holding off on a pitch on the paint, as evidenced by a nearly two percent drop in whiffs on those offerings from 2016. With the slider, it seemed to induce more swings.
If Diaz is going to throw the fastball so much, then the obvious tweak he needs appears to be with that offering. But what if the Mariners looked at what Diaz has done best in his time in the Majors, and tried to amplify it?
Overall, Diaz’s fastball hasn’t been terrible. But it hasn’t been good, either. By wOBA, it ranks 137th out of 354 pitchers in the last two years. It was beaten up by righties in 2016 and then lefties in 2017. Even if the year-to-year stickiness of those numbers isn’t necessarily reliable, the real hammer has always been the slider. It’s yielded a meager .187 wOBA. By expected wOBA, Statcast actually says it’s even been 22% better than that. Diaz simply upping its usage would likely bring more whiffs for him. The pitch generates a greater percentage of swings and misses (33.8) than the fastball gets misses and called strikes together (30.4).
There’s also this: Diaz throws the slider 15% less to lefties than to righties, who have also hit his fastball harder and more consistently. He has room to use it more against opposite handed hitters, and doing so seems like a natural progression.
Beyond that, there might be two things Diaz could tinker with in regards to his breaking ball that could enhance his overall game. He primarily pounds the low, glove side corner of the zone with it. Commanding the pitch to additional parts of the zone — say, in the vein of Kenley Jansen’s cutter — would force hitters to attempt to be more accountable to it, while still being subjected to its devastating drop. This could pair really well with a more erratic fastball, too. If a batter has to be aware of the slider breaking in different portions of the plate, they could be coaxed to swinging at a wilder heater coming at them 10 mph faster.
While it would require more sophistication and time, Diaz could also adjust his arm slot for his slider depending on the handedness of a batter to give it a different look. This may come with more caveats than benefits at first. Max Scherzer has said this kind of approach takes years to master. Zack Greinke has suggested it provides one globby, less useful look more than two distinct ones. And of course, Diaz has already been cited as having control issues at times. But the fact of the matter is he’s young and immensely talented, and finding ways to make his slider more of a weapon should be a priority. It could be what makes his potential dominance undeniable.
Feature image from Eric Hartline, USA Today Sports. Data from Statcast; gif from PitcherList.