Change for the worse: Are Strasburg's struggles tied to his changeup?

What's behind Stephen Strasburg's struggles this year? The answer may lie in his changeup, which has seen a dramatic dip in effectiveness in 2015.

After starting the year a surprising 7–13, the Nationals have quickly turned their season around. Starting with a come-from-behind 13-12 win over the Braves on April 28, Washington has won 11 of its last 13 to improve to 18–15, going from last place and eight games out in the National League East to second place and just 2 1/2 games behind the Mets entering play on Tuesday. But as dramatic as that improvement may seem, it has not been complete. Outfielder Jayson Werth is still scuffling at the plate. Third baseman Anthony Rendon’s anticipated return from a left knee sprain has been delayed significantly by an oblique strain. And Stephen Strasburg, who will start Tuesday night against the Diamondbacks, is still searching for his second quality start of the season.

Strasburg struggled through a rough April, posting a 4.60 ERA in five starts, only one of which—a home outing against the lowly Phillies on April 19—was quality. But concerns about the 26-year-old righthander increased dramatically with his first start in May, which came at home against the Marlins on May 5. It took Strasburg 64 pitches to get through three innings in that game, after which he was removed due to discomfort behind his pitching shoulder. That discomfort was reportedly just a cramp, and after a chiropractic adjustment and a few throwing sessions, he is said to be good to go for Tuesday.

Still, Strasburg will take the mound in Phoenix having allowed eight runs in his last 14 1/3 innings, and he now sports a 4.73 ERA on the season. Given the recent injury scare, one wonders if there’s more to his performance than some bad luck and a slow start.

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To be sure, bad luck has played its part, as opposing hitters are batting .398 on balls in play against Strasburg. On the other hand, he's had good luck on fly balls. With his sinker withering on the vine (he's thrown it just 2.3% of the time this season, and the average speed of the pitch is down nearly two miles per hour from a year ago, per BrooksBaseball.net), Strasburg is giving up fly balls at his highest rate since 2012. However, while 7.4% of those fly balls were home runs in 2012, just 2.0% have been hit out of the yard this season. In general, Strasburg has been typically effective in limiting extra-base hits, but he is allowing plenty of hard contact. His line-drive rate this season has been a career-high 28%, per Baseball-Reference.com, and it would seem to be only a matter of time before that hard contact and those fly balls yield home runs with more frequency.

Meanwhile, Strasburg’s strikeout rate is significantly below his established standard. Over the last four seasons, Strasburg has struck out 28% of the batters he has faced. This year, that is down to 21.3, which could be related to his continued, albeit gradual, decline in fastball velocity. There has indeed been a corresponding decline in the percentage of hitters swinging and missing at Strasburg’s four-seamer (from 9.0% in 2012 to 5.2% this season, per Brooks), but even more dramatic has been the reduced effectiveness of his two primary off-speed pitches. Prior to this year, Strasburg had never recorded a swing-and-miss rate of under 23.4% on his changeup. This season, opponents are missing that pitch just 15.7% of the time. Likewise with his curveball: Last year, it got a swing and a miss 15.1% of the time, but this year, it’s plummeted to 6.5%.

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The reduced effectiveness of his changeup might be the biggest concern. Over the last five seasons, opponents' line-drive rate on that pitch has climbed, from 17.4% in 2010 to 27.8% this year. Strasburg’s changeup is widely considered his best pitch and has in the past been one of the most dominant pitches in the majors, but this year, opponents are hitting it harder, more often and for more power than ever before. One reason for that could be the declining velocity of his fastball. The gap in speed between Strasburg’s fastball and changeup has been about seven miles per hour ever since he returned from Tommy John surgery and remains so this year. But with Strasburg no longer threatening to touch triple digits with the former pitch, it may be may be easier for opponents to adjust to his 88 mph changeup.

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Strasburg’s fastball has topped out above 99 mph every season of his career, but this year, he has yet to break 98 with the pitch, which has averaged a career-low 95.3 mph. That is a crucial difference. Having to protect against near–triple-digit heat forces a hitter to cheat and start his swing early, making it all the more difficult to adjust to a high-80s pitch thrown with a nearly identical delivery. But if Strasburg’s opponents aren’t protecting against that elite velocity, they’re more likely to be able to wait on the changeup.

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Strasburg’s changeup still has filthy movement—15.7% is a high whiff rate for any pitch—but batters aren’t finding it unhittable like it has been in the past. On the season, Strasburg’s opponents have hit .250 with a .393 slugging percentage against his change, up from .155/.226 last year. The important figure there is the resulting isolated slugging: It's increased from .071 in 2014 to .143 this year, proving that opponents are hitting the pitch harder.

Reduce the effectiveness of a pitcher’s best pitch, and there’s little doubt that his performance will decline with it. The question heading into Tuesday night’s start, then, is less about the health of Strasburg’s shoulder than about whether or not he can make the necessary adjustment to restore the dominance of his changeup or to compensate with the rest of his arsenal.