A look at the 10 most improved pitchers this year

We're about 40 percent of the way through the season. Baseball's top starters have made more than a dozen starts, and have thrown nearly 100 innings. Just as we did last week with hitters, it's time to check out 2018's most improved pitchers.
How, though? You could look at ERA, or strikeouts, or walk rate, or any number of things. (Not wins. Never wins.) Here at the Statcast™ lab, we like to try to exclude the effects of defense or ballpark and to just look at mastery of the hitter/batter relationship. We want to find pitchers who have limited both amount of contact (and walks) and quality of contact, in terms of exit velocity and launch angle.
That's what our "Expected wOBA" metric does. (Read more about how it works here.) It looks at the combinations of strikeouts, walks, exit velocity and launch angle, and outputs a number that's similar to OBP, just with more credit given for extra-base hits. (The 2018 Major League average is .331.)
So let's look at the 131 pitchers who faced at least 200 hitters last year and at least 150 this year. That's actually slightly more than the 90 pitchers who are "qualified" by traditional metrics, which gives us a larger sample. (Sorry, Miles Mikolas. You've been great, but you didn't pitch in the Majors last year, so you don't qualify here.)

Here are your 10 most improved pitchers so far -- and our best guess at a reason why.
Justin Verlander, Astros
-.088 (down from .317 to .229)
Likely reason: New home / Mechanical change
It's not surprising that Verlander, the easy front-runner for the American League Cy Young Award, is at the top of a list of successful pitchers. It is surprising that he's atop a list of most improved pitchers, because he was already really, really good last year. You wouldn't think there was that much else Verlander could have done.

You'd be wrong, because Verlander has stepped it up in every way. He's getting more strikeouts, allowing fewer walks and home runs, which is pretty much everything a pitcher needs to do. There's some evidence that a minor mechanical change to raise Verlander's release point has allowed his fastball and his slider to be even more effective. Whether or not that can be credited to the impressive Houston analytical group, he's been baseball's best pitcher this year -- and the most improved.
Tyler Glasnow, Pirates
-.074 (down from .376 to .302)
Likely reason: Shift to bullpen
A change in role is an easy reason for improved performance, and it's a pretty easy reason for Glasnow's success. Last year, he had a 7.69 ERA in 15 games, 13 as a starter, which was a factor in him opening 2018 in the Pittsburgh bullpen. While Glasnow's 4.89 ERA this year isn't impressive, it's also misleading; 10 of the 19 runs he's allowed this year came in just two games.

Instead, Glasnow has increased his strikeout rate from 18 percent to 29 percent, while lowering his walk rate from 14 percent to 11 percent (which is still too high). Like many starter-to-reliever conversions, he's benefited from increased velocity (his fastball is up from 94.6 mph to 96.3 mph) and a more favorable pitch mixture. Glasnow has dumped his ineffective sinker, prioritized that blazing four-seamer, and added a solid new slider.
Vince Velasquez, Phillies
-.070 (down from .364 to .294)
Likely reason: Health, probably
Velasquez's first few seasons in the pros were marked by flashes of talent around inconsistency and injury, beginning with Tommy John surgery in 2010. With the Phillies in '16, he missed time with a right biceps strain; it '17, it was a right flexor strain in May and season-ending finger surgery in August.
The latter injury was later amended by the pitcher himself to have been the far more serious thoracic outlet surgery, where a rib must be removed, but it appears to have gone well. "I can say this is the healthiest I've been in five years," Velasquez told local media in May.

It's difficult to quantify how a pitcher is feeling, but the results are clear. Velasquez has upped his strikeout rate from 22 percent to 28 percent, and he's done it while dropping his walk rate from 11 percent to eight percent.
Seth Lugo, Mets
-.066 (down from .330 to .264)
Likely reason: Shift to bullpen
Robert Gsellman, Mets
-.055 (down from .335 to .280)
Likely reason: Shift to bullpen
We might as well lump these two together, because they're very similar stories. Lugo and Gsellman had started 55 combined games for the Mets in 2016 and '17, with varying amounts of success.
This year, they landed in the bullpen, though Lugo's success as a spot starter may keep him in the rotation for now. You've seen this movie before: Lugo added 2.6 mph to his fastball, the second-largest jump of any pitcher aside from Cincinnati's Amir Garrett, another bullpen conversion. Gsellman didn't add quite as much, but went from 92.8 to 93.4 on his sinker.

Lugo, meanwhile, jumped the usage of his high-spin curve from 17 percent to 31 percent, the second-largest jump behind Atlanta's Lucas Sims. Gsellman went all-in on the sinker. In 2017, he threw it 45 percent of the time, mixing in 15 percent four-seamers. Now you see the sinker 56 percent of the time, with the four-seamer all but gone.
"More velocity and throwing your best pitches more" is pretty much the handbook for successful starter-to-reliever conversions, and these two are playing it out perfectly.
Ross Stripling, Dodgers
-.052 (down from .282 to .230)
Likely reason: Change in approach
Stripling spent his first 10 games of the year in relief this season, and he was more good than great. After a poor spot start on April 30, he was back in relief, returning to the rotation due to injuries on May 6. The way Stripling has pitched since, he may never leave. In seven starts since, the numbers are unbelievable: 40 2/3 innings, a 1.33 ERA and 53 strikeouts against just four walks.

It's not just the whiffs, either. Of 148 starters who have allowed 100 balls in play, Stripling's 21.6 percent hard-hit rate is the lowest. What's going on here? Stripling reportedly was told to throw his curveball harder, though that hasn't really shown up in the data. Instead, this appears to be about effective use of his four pitches; only four regular starters have thrown fewer fastballs.
Sean Newcomb, Braves
-.051 (down from .332 to .281)
Likely reason: Pitch changes
Newcomb's problem, throughout the Minors and into the big leagues, has been an inability to throw enough strikes. Believe it or not, that hasn't actually changed, but he's still having a really strong season. Why? In part thanks to improving and increasing Newcomb's changeup, which has helped give hitters another look beyond just a fastball and curveball.

Now, Newcomb is just outside the Top 10 for lowest hard-hit rate among starters. Now, he's getting 49 percent grounders, up from 43 percent. Newcomb still doesn't throw enough strikes. It hasn't stopped a legitimate improvement.
Gerrit Cole, Astros
-.050 (down from .320 to .270)
Likely reason: A new home
You know this story by now, right? As soon as Cole was traded to Houston, every corner of the baseball world shouted that he'd throw fewer flat sinkers and more breaking pitches and improve. That's exactly what happened. Cole has all but dumped his changeup, and he's throwing his sinker far less. Last year, he threw his slider and curve 29 percent of the time; this year, it's over 40 percent of the time.
It's a little more complicated than that, of course, but the results have been spectacular. Cole has a 38 percent strikeout rate, the second best of any starter behind Max Scherzer, and an enormous increase from his 23 percent last year. In 2015, the Pirates thought they'd found their next ace. It didn't work out the next two years. In '18, with Houston, he's been better than anyone could have thought.
 Tyson Ross, Padres
-.048 (down from .377 to .329)
Likely reason: Health, and a new home
Ross was once a reliable starter for San Diego, but he threw only 5 1/3 innings in 2016 after injuring his shoulder on Opening Day, then got into only 12 games for Texas last year -- with a 7.71 ERA -- after recovering from thoracic outlet surgery. That lousy 2017 performance, where he walked 37 against just 36 strikeouts, didn't exactly make improving in 2018 a high bar to clear.

Still, a healthy Ross has been legitimately good this year. In 27 fewer innings than last year, he's already struck out 42 more hitters than he did in all of 2017. This isn't a story about velocity, however. Ross has given up on his sinker, instead using more sliders and cutters, leading to more whiffs.
Mike Foltynewicz, Braves
-.044 (down from .340 to .296)
Likely reason: Sharper pitches, more velocity, more sliders
Like his teammate Newcomb, Foltynewicz has taken big steps forward this year to overcome inconsistency and develop into a top-flight starter. He's upped his strikeout rate from 21 percent to 29 percent; he's allowed only 0.57 homers per nine after giving up 1.19 per nine last year.
There's some proof that Foltynewicz sharpened his pitches this year, gaining more separation between his fastball and his slider, and he talked about simplifying his mechanics this spring. But whatever the underlying reasons are, the outcomes have been clear. He's throwing his fastball harder, up from 95.2 mph to 96.3 mph, and he's throwing a good slider far more, at the expense of a relatively unimpressive sinker.

Just missed:Rick Porcello; Jesse Chavez; Kyle Gibson; Derek Holland; Zach Eflin; Trevor Bauer