One calendar month into the 2015 season, much has been said about the starting rotation of the St. Louis Cardinals. Heck, with a record of 20-6 (.769 winning percentage), much can be said about the Cardinals as a whole. If anything, the offense, bench, and bullpen have been terrific at picking up the starting rotation, with short starts (less than five innings pitched) by Tim Cooney, Carlos Martinez, and Tyler Lyons in the last week.
Thus far, Michael Wacha has been the "horse" of the rotation, averaging more than 6 & 1/3 innings per start. While his 1.93 ERA is sixth lowest in the National League, his 3.63 FIP lags behind at 30th. This disparity suggests his ERA is likely unsustainable, and a major contributing factor is his decreased strikeout rate. Another measure to keep an eye on, especially if his strikeout rate remains low, is his left-on-base percentage (LOB%). Currently, it is at 87.0%, fourth highest in the National League, and given the MLB average was 73.0% last season, he is probably due for some regression.
As you can see, throwing first pitch strikes is not an issue as it has improved in each of his first three MLB seasons. This is one of the first things I look at because I am curious whether or not the pitcher is consistently putting himself behind in the count. Wacha's swinging strike rate is down to a career-low 7.7%, and hitters are making a considerable amount (74.0%) of contact on pitches outside of the zone (2014 league average: 65.8%). All of this adds up to a strikeout rate of 13.4% (or 4.96 strikeouts per nine innings).
Now, being five starts into the season, is this reliable and/or something to expect going forward? Maybe, but probably not. Russell Carlton of Baseball Prospectus provides "stabilization points" for when we can start considering certain statistics reliable. Regarding strikeout rate, Carlton's research shows that it stabilizes after 70 batters faced, and Wacha has faced 134. That being said, ZiPS (rest of season) still projects Wacha's strikeout rate to return to near-2014 levels at 7.87 K/9 IP.
...so what's going on?
To be honest, it's complicated. The first thing most jump to is fastball velocity. It has to be way down, right? After all, he is coming off a unique shoulder injury, and it just may be in his best interest to harness velocity for longevity purposes. While his average fourseam fastball velocity has decreased from where it was in 2014, it has not been an Earth-shattering difference at 0.34 MPH. Yet, for a fastball-changeup pitcher, a 0.34 MPH decrease can be a contributing factor. However, as you can see in this April 11th strikeout of Devin Mesoraco, Wacha still is able to dial up the fastball (to 96-97 MPH) when he wants, especially if he is using it to set up an offspeed pitch.
What is particularly weird is that Wacha's whiffs per swing are actually increased from 2014 for three of his four pitches. The problem is that the one pitch in which he has seen a decrease is his fourseam fastball, and he just so happens to throw this pitch ~52% of the time. It is not a slight decrease, either. In 2014, hitters were missing on 24.2% of their swings; so far in 2015, it's down to 13.56%. With two strikes on the hitter, 2015 Wacha has not wavered from his 2013-2014 approach, as he is still going with the fourseamer ~50% of the time, versus both lefties and righties. Thus, one could argue that his fastball is not serving as the put-away pitch it was in previous seasons.
Another component to consider is his increased cutter use. Prior to 2015, he used the cutter 6.41% of the time. In 2015, he has thrown it nearly 20% of the time. Unlike a slider, a cutter is used to induce weak, on-the-ground contact much more than swings and misses. Sure enough, 62.07% of Wacha's cutters that have been put in play have been on the ground, and it has resulted in only one extra base hit (a double) thus far.
The final thing I will address is the average pitches per plate appearance. So far in 2015, hitters are averaging 3.47 pitches per plate appearance versus Wacha, as compared to 3.78 pitches per plate appearance in 2014 and 3.97 in 2013. To me, this could signify a change in approach. Wacha, confident in his stuff, is not afraid of hitters putting his pitches in play. So far, it seems to be working as his ground ball rate has skyrocketed to 54.0%, and he has limited hard contact to 22.4%, the 9th lowest in the National League.
Should we be worried? Probably not, but at the same time, don't be expecting a sub-2.00 ERA for much longer, especially if his strikeout rate remains down. While Wacha has a tremendous infield defense behind him, these ground balls are eventually going to find holes, and he will be put in high-leverage situations where he needs strikeouts. Don't worry, the stuff is still there to get these strikeouts, he just may need some minor tweaks to his pitch usage and sequencing, something I am confident will be addressed by Yadier Molina, Derek Lilliquist, and the other pitchers on staff.