In case you haven't been watching the Dodgers for the past year and a half, Clayton Kershaw has ridiculous stuff. His fastball is one of the hardest in the major leagues and his curveball moves about as much as any you'll ever see. His stuff is so good that even when one of his pitches isn't working he is still nearly unhittable. How good is his stuff though? One way to measure it is using a stat called wFB/C.
The wFB stat weighs how many runs a pitcher allows compared to an average pitcher when they throw a fastball. There are similiar stats for each pitch, wCB for weighted curveballs, wCH for weighted change-ups, and so on. But wFB/C weighs the amount of runs a pitcher allows per 100 fastballs compared to the average pitcher, so it's a rate stat instead of a cumulative stat. So if a pitchers fastball is 0 wFB/C, that means it's perfectly average. If it is a 1, that means the pitcher allows 1 less run per 100 fastballs then an average pitcher would. If it is a -1, that means a pitcher allows 1 more run per 100 fastballs then the average pitcher would. Here's a chart ranking qualifying pitchers based on their wFB/C:
Using this chart we can see that Kershaw's fastball is fifth best in the major leagues in terms of its effectiveness. For every 100 fastballs Kershaw throws he allows 1.5 runs less than the average pitcher does, which is really good. Keep in mind that this only measures the effectiveness of fastballs, not how hard they are thrown. That helps explain the first two players, Chris Carpenter and Dan Haren, who both throw had but also have great command of their fastballs, which is what makes them so effective. They also have great secondary pitches which help set up their fastballs (Carpenter has a great curveball while Haren relies on his split-finger).
Tim Wakefield may be a surprise, but when you remember that he only throws his fastball 9% of the time it's not surprising that he would be effective with it. Jarrod Washburn doesn't have a blazing fastball either but he has great control, good offspeed pitches, and a great outfield defense that has helped him thrive in spite of his flyball tendencies.
But right after them comes Clayton Kershaw. To me it is really remarkable that his fastball would be this effective with the control he has. So often during his starts he has trouble getting his curveball over and has to rely almost solely on his fastball, yet it is still so good that hitters cannot square it up. For comparison here are the five least effective fastballs:
All five of these players have ERA's above 5.00. They have all had huge control problems for at least a part of 2009 and none of them throw particularly hard (Liriano hasn't thrown hard since his arm injury in 2007). Part of this is bad luck, obviously Ricky Nolasco does not have the second worst fastball in the major leagues, but their poor usage of their fastball explains much of these pitchers struggles in 2009.
What about curveballs? Where does Clayton's curveball rank above the game's elite? Unsurprisingly it's pretty high, 13th in the National League in fact:
Most of the names on this list aren't too surprising. Guys like Javier Vasquez, Aaron Harang, Barry Zito, and Chris Carpenter are knon for their devastating curveballs. The American League list is similiar with guys like Roy Halladay, Josh Beckett, and A.J. Burnett among the leaders. Pitchers like Randy Wolf and Wandy Rodriguez also have nice hammers that they use as their out pitch. It's a bit surprising that Kevin Correia is ranked so high but he has been surprisingly not terrible this season. There's got to be something in the water in St. Louis as four of their starters are on this list.
And then we have Kershaw, who is again among the leaders. What keeps him from being higher on this list is not the movement on his curveball, but his control of the pitch. You can bet that as soon as he learns how to control his curve it's going to be as good as anybody's. His changeup is still not a very good pitch so his curveball is still what he relies on to offset hitters timing. His changeup is -1.91 runs per 100 pitches, pretty bad actually. He ranks 69th or 90 qualifying pitchers in that category. So unless he can drastically improve there his fastball and curveball will remain the only pitches he can throw very often.
What all this means is hard to say. His fastball is already as good as nearly anyone's in the world all things considered. His curveball is extremely effective as well, although there is still room for improvement. His changeup still has a long ways to go but even without it he still has an ERA under 3.00. What I do know is that if he ever learns how to harness his command of his offspeed pitches he is going to be something to behold, and it's certainly going to be fun watching him try.