Through four starts, the reigning AL Cy Young winner sports a 13.50 ERA. He has walked 14 and struck out 14 in 18 innings. Last season, in 241 regular season innings, Sabathia walked only 37. The season before, he walked 44 (in 192 innings). So what’s wrong?

According to pitch f/x data, Sabathia’s velocity is fine: although in one start he was throwing 92-93, in his other starts his fastball regularly clocks in between 94-96 MPH, and he appears to maintain his velocity throughout the games (granted, he hasn’t lasted very long in any start, but he has thrown a lot of pitches). A drop in velocity would be a huge red flag signifying a potential injury; however, his velocity seems to be fine.

Additionally, Sabathia’s stuff seems to be okay – at least on the surface – as evidenced by his 14 strikeouts in 18 innings (seven Ks per nine innings). Last year, he struck out 7.80 hitters per nine, and the season before he Ked 8.06. Yes, his strikeout rate this year is a little lower than the previous two seasons, but this can possibly be attributed to a small sample size – if Sabathia struck out one more hitter this year, his K rate would jump to 7.5 per nine. His strikeout rate is key to watch going forward: if it does not regress towards the level he established during his previous two seasons, it could be a sign of decreased “stuff,” which could go hand-in-hand with an injury.

Sabathia’s batting average on balls in play (BABIP) is an abysmal .415. His strand-percentage is an atrocious 50%. He has also given up a lot of homers per fly: 20% of all fly balls he’s allowed have become homers (as opposed to 8.6% last year and 9.8% the tear before). All of these numbers are much, much worse than any previously established level. But are they simply aberrations, or side effects demonstrating that Sabathia has been a different pitcher this year?

Sabathia’s biggest problem this year is his inability to throw as many strikes as in the past two years. In 2005, Sabathia threw 69% of his pitches for strikes; he followed this up by throwing 66% of his pitches for strikes in 2006. This year, however, he’s only throwing 63% of his pitches for strikes.

Additionally, Sabathia is inducing fewer swings-and-misses this year than he has in the past. So far this year, batters have swung at missed at Sabathia’s pitches 14% of the time. Last year, they swung and missed at 18% of all pitches; the year before it was 17%. Along these same lines, Sabathia is relying on called third strikes for his strikeouts: this year, 36% of his strikeouts have been called third strikes; last year it was 23%, the year before it was 20%.

Sabathia has also given up a lot of hits on 0-2 counts. He has gotten as many 0-2 counts as in previous years (24% of all at-bats have had an 0-2 count this year, compared to 21% last year and 23% the year before). However, last season, batters got a hit on an 0-2 count 14% of the time. The year before it was 7% of the time. This year batters have gotten a hit on an 0-2 count a whopping 31% of the time.

But perhaps the most interesting thing about Sabathia this year is his pitch selection. This wonderful blog by Josh Kalk keeps track of pitch f/x data for pitchers. As you can see, last season Sabathia threw a fastball or sinker 61% of the time, a changeup 17% of the time, and a slider 22% of the time. And in his first start, against the White Sox, these trends continued: CC threw 68% fastballs, 11% changeups, and 21% sliders. Yes, he threw fewer changeups than normal, but I don’t think that’s of much importance. The key here is that he threw approximately the same amount of sliders. And he also induced 14 swings-and-misses.

However, in his second start, against Oakland, something changed. Sabathia threw a slider only 14% of the time (and of the sliders he did throw, 40% were thrown to one hitter, Daric Barton). In this start, Sabathia induced only two swings-and-misses. 

This trend continued in his third start, also against Oakland. Sabathia threw his slider only 15% of the time, and once again 46% of his sliders were thrown to Daric Barton. In this start, batters swung and missed only four times.

In his most recent start against Detroit, Sabathia threw his slider only 8% of the time. In this start, he was able to get swings-and-misses only four times.

In summary: Sabathia’s velocity appears to be fine. However, he is throwing fewer strikes. Of the strikes he is throwing, fewer are swing-and-miss strikes, but rather called strikes instead. In the one start this season in which he induced a lot of swings-and-misses, he threw his slider the same amount as last season. In the following three starts, he induced a total of 10 swings-and-misses, and he threw 10% fewer sliders (and 34% of these sliders were thrown to only one hitter, Daric Barton). His line-drive percentage is up and he is giving up more homers per fly ball – in other words, batters are hitting him very hard.

In his own words, his problem is “not mechanical. My velocity is fine. I just can't command both sides of the plate.”  ESPN’s Keith Law agreed, saying in a recent chat: “His fastball command is in the toilet. Last night he couldn't come across to his glove side with his fastball unless he forced it all the way in behind a right-handed hitter. When he threw a FB for a strike, it was practically on a tee.”

If the problem is indeed command – especially with his fastball – why is he throwing a disproportionate amount of fastballs? This year, 67% of all of his pitches are fastballs, as compared to 61% last year. He’s also throwing a few more changeups – 19% this year as compared to 17% last year. Sabathia has simply stopped throwing his slider as often: he has thrown only 14% sliders (and if you remove the first start of the season, Sabathia has thrown only 12% sliders).

As much as I hate to say it, there is one potential answer to this question. Let’s say you’re CC Sabathia, pitching in the last year before you are set to earn a bagillion dollars on the free agent market. Perhaps last season, perhaps in spring training, or perhaps in your first start of the season against the White Sox, you feel something strange in your arm or shoulder. Maybe it’s tingling. Maybe it’s pain. Maybe it’s just uncomfortable. But no matter what it is, you only feel it (or it’s exacerbated) when you throw an off-speed pitch. Like a slider. What do you do?

You stop throwing as many sliders. And other teams sit on your fastball. And if you can’t command the fastball (which may or may not have to do with that strange feeling in your arm), you walk guys and get hit hard.

Now, I’m not saying CC Sabathia is hurt. It’s quite possible that he is simply experiencing bad luck. It’s quite possible that he’s lost his confidence in his slider and just doesn’t want to throw it.

But CC Sabathia has an extensive track record. Over the last two seasons, he was almost the exact same pitcher. Even in 2005 he was very similar to his 2006 and 2007 self. Yet after throwing 256 innings last year between the regular season and playoffs, Sabathia is suddenly throwing fewer strikes, inducing fewer swings-and-misses, giving up more hits on 0-2 counts, giving up harder hit balls and more homers per fly balls, and throwing his slider noticeably less than last season.

Certainly makes you wonder.