This is good analysis but if I'm reading correctly, he's choosing his individual pitches with about the same frequency last year and this. There's the 0.7 MPH drop in fastball velocity and the 0.2 increase in change up speed but both numbers reflect the entire season. Up until 2 weeks ago though, he was still getting guys out.
Aside from poor location, what's the biggest difference between the last 2 weeks and Johnson's body of work all of last year and the early part of this year?
It comes down to a long twitter conversation I had with one of the guys over at BSR.
While he is throwing his changeup and curve about the same as last year (throwing each 2% less this season, which is minimal) the problem is that neither of them are anywhere near the zone. In May his Curveball has only been a strike 50% of the time, and he's only throwing it once or twice an outing. So it's essentially a non-factor.
That leaves him as a 1-pitch pitcher. You can get away with being a 1-pitch guy as a reliever and not changing speeds IF you have the command to get ahead of hitters and locate your pitches. On the other side, command is less important if you are able to change speeds effectively and keep hitters off balance. Basically, you can survive without 1 of the 2 (more or less), but Johnson doesn't have the ability to command his pitches OR change speeds.
That's where a lot of this comes in. He's falling behind, and having to lay it in the zone just so he doesn't walk batters. He's not keeping the ball down as much, so it's getting hit harder, hence the increased LD% that Rob mentioned above.
Obviously, command is an issue; we can all see that when watching the games. I was just hoping to point out the difference between this season where he's getting hit hard, and last year. Last year, when his command was off, he was able to change speeds more effectively and limit the damage. This season he hasn't been able to and hitters are taking advantage.