A tale of two seasons for St. Louis Cardinals reliever Kevin Siegrist

As a rookie in 2013, Kevin Siegrist was a rock out of the bullpen for the St. Louis Cardinals—posting a 0.45 ERA (lowest in MLB, minimum of 30 IP), a .128 batting average against (lowest in MLB), and a 98.3 LOB% (second highest in MLB) in 39.2 innings pitched. Unfortunately, this high-level performance did not carry over to his sophomore season in 2014. In fact, for relievers with at least 30 innings pitched, his 6.82 ERA ranked third highest, and he wasn't nearly as efficient stranding runners as his 62.8 LOB% was the 11th lowest.

So, what was different for Siegrist in 2014? Sure, he dealt with arm soreness during spring training and then suffered from one of those infamous throwing arm nerve injuries (spanning two months, in which he missed 52 games). However, prior to being sidelined with the nerve injury, when he was reportedly healthy, his performance was still startlingly bad.

When I have discussed the Siegrist issue on Twitter, the most common response has been that he suffered from a decline in fastball velocity. This isn't necessarily unwarranted as apparently Siegrist himself mentioned lacking "explosion" to the media last season. However, the following table shows that his average fastball velocity in 2014 saw only a slight drop (less than one MPH) from where it was in 2013. In fact, it was the same to start and end the season, with the drop occurring in the time immediately following his return from the disabled list in July (not unexpected as he got back "up to speed"). While we are at it, his fastball's dragless horizontal movement, vertical movement + gravity, and vertical release point weren't noticeably different either:

His most-used pitch is clearly his fastball, so let's take a look at some of its results from the last two seasons:

Batting average against fastballs

As you can see by the blue dominating the graph, hitters were only able to muster a .188 batting average on fastballs in the strike zone in 2013. Yet, in 2014, hitters mashed their way to a .381 batting average on fastballs in the zone. For a pitch that Siegrist went to 87.43% of the time (a ~2.7% increase from 2013) in 2014, this is troubling. The success on balls in play by hitters is especially telling as both the whiff rate (2013: 10.26%, 2014: 10.32%) and whiffs per swing (2013: 22.87%, 2014: 22.54%) remained roughly identical between the two seasons. This means that though hitters are swinging and missing on his fastball at the same rate, they are hitting the ball better (and subsequently enjoying more success) when they do make contact:

Fastball results (overall)

Up to this point, we have decided that velocity, movement (both horizontal and vertical), and vertical release point are not the culprits for Siegrist's extravagantly unsuccessful 2014 season. This leaves "location, location, location."

Fastball location

Unfortunately, location does not really seem to be the issue here, either. Sure, he had a slight bump in middle and upper zone pitches compared to 2013, but as you can see from this 2014 ISO graph, he was hit hard down and up last season. Plus, for what it's worth, Siegrist "grooved" fewer fastballs in 2014 than he did in 2013.

Bottom line

Siegrist's 2014 could always be chalked up to "reliever volatility," but the stark difference in performance from 2013 to 2014 despite having virtually identical processes (see above) makes this reason slightly unreasonable, in my opinion. The next reason, as Eric talked about on Twitter, could be that hitters were able to adjust to Siegrist's deceptive delivery as experience and game film grew. This certainly is plausible (and I reason I probably agree with), but it is hard to substantiate because even if we had access to hitters who have faced Siegrist, there is no chance they would tell us this because they wouldn't want Siegrist to make adjustments.

One thing is for certain, though: the potent fastball Siegrist rode in 2013 was almost exclusively ineffective in 2014. Pitch values are always a tricky subject, but the results seen above provide backing to the fact that his fastball went from 11.8 runs above average in 2013 to 4.9 runs below average in 2014. Thus, without even discussing his arm health, it is safe to say I'm worried about Siegrist's future. Unless he gains confidence in his secondary offerings, hitters will be able to "sit" on his fastball and won't have to work through the deception he benefited from his rookie season. What does the future hold for the 25-year-old lefty? Hopefully his arm is healthy enough for us to find out.

Credit to BrooksBaseball, Baseball Prospectus, and Fangraphs for the data used in this post.