Is he really injured? No, probably not. But it's the way you save face for pitchers doing badly that you don't want to outright cut.
Fancy Stat Time!
BABIP (ranges between .250 - .325)
Career: .292 This year : .319
Career: 10.8% This year: 15.5%
Career: 72.9% This year: 67.9%
On some level you want to believe he should turn more toward the averages, both of his career and baseball in general, and be better. But these things aren't wildly off. Ok, maybe a few more balls stay in the park, but the BABIP, the LOB%? These could easily be the sign of a bad pitcher, too. They are in the range so it's not like it HAS to come back down.
Career 7.58 / 1.88 This year : 7.09 / 1.49
Career : 42.8% This Year : 34.9%
The strike-outs and the walks might fool you. His control seems great and he's striking out enough guys not to be worried about it. But the GB% tells the story. He's not fooling anyone here. He can't get anyone to hit the ball down anymore.
Why is that? Ben Duronio at Fangraphs explains it nicely. Nearly everything Dan Haren throws is in the upper 80s. Guys can time pitches when they have the speed down. They may be fooled still by a pitch with good movement but they aren't going to be off-balance anymore. They are just hacking away at what Dan throws (11th in pitches swung at by opponent). We talked about this earlier but in 2011 when his FB velocity started to come down, he moved to a cutter that was devastatingly effective. At the time his FBs averaged 4.7 MPH faster than his cutter. Now it's 3.4.
To show you in another way how much pitch speed matters, here are two pitchers stats for balls (O-swing is precent of times the opponent swings at pitches outside the strike zone, Z-Swing is inside, others should be self-explanatory)
O-Swing 32.7%, Z-Swing 67.2%, Swing 49.5%, Contact 81.5%
O-Swing 34.6%, Z-Swing 63.5%, Swing 50.0%, Contact 82.6%
OK Guy #2 here is doing a little better perhaps because guys are swinging at more outside stuff and less inside stuff, but the numbers are pretty similar enough that you'd expect them to be relatively the same.
Guy #1 is Dan Haren
Guy #2 is Cliff Lee
Now check out pitch mix and (speed)
FASTBALL - 37.0% (89.2), CUTTER 40.4% (85.8), CURVE 3.7% (77.6), SPLIT FINGER 18.9% (84.8)
FASTBALL - 57.9% (90.7), CUTTER 19.5% (86.9), CURVE 8.1% (74.5), CHANGE 14.1% (84.2)
Cliff Lee's cutter is 3.8 MPH slower, Haren's is 3.4. Not too different. His change is 6.5 MPH slower, Dan's split is 4.4. That's a big difference. Lee's curve is 16.2 MPH slower. Haren's curve is 11.6. Big difference again. What's more is that Lee actually thows that curve more than twice as often as Dan throws his. Toss in the occasional 81MPH slider and Lee has a repertoire that can keep a hitter unbalanced. It doesn't matter that they can "hit" Lee because they aren't hitting him well. The same can't be said of Haren.
This stuff matters as much as pure velocity. Jamie Moyer in his Phillies years had a fastball that averaged around 81MPH but because he could work in a 75 MPH chance and a 69 MPH curve 25-30% of the time he kept hitter off balance enough to survive.
Why not just pitch everything else slower? Sorry - doesn't work that way. For almost every pitcher you throw pitches at speed X. That's where you feel comfortable. That's where it works for you. That's where you have the movement and control you like. You've learned this over years and years of pitching. You can't just say "oh I'm going to throw my change 2 MPH slower because I lost 2MPH on my fastball."
Could Haren throw the curve more? Perhaps. But it's never been a great pitch for Haren. It would require him to be a different type of pitcher. One that doesn't look to set up a great cutter or slider or fastball, but instead really pitches to every batter never relying on a single pitch to get him out of trouble. It's a different mindset really. And he'd need time to figure out how to do this. Say 15 days at least, maybe 60.
Here's a nice visual interpretation of pitch speed differences from Brooks Baseball.