When it comes to Tim Lincecum's struggles, the obvious explanation is that he's having a hard time because his fastball velocity is down.
It is indeed. Per FanGraphs, Lincecum's average fastball velocity is down from 92.2 mph in 2011 to 90.1 mph this year. With a fastball like that, you can't make mistakes, and Lincecum has made a lot of mistakes.
The bright side is that Lincecum is still striking out hitters, as he has a K/9 of 9.79. The not-so-bright side is that he's having a very tough time putting hitters away. He's averaging a career-high 18.5 pitches per inning, and he's walking close to five batters for every nine innings pitched.
Back in April, Dave Cameron of FanGraphs wrote that it was too early to assume that fastball velocity would be Lincecum's biggest problem all season, as pitchers' struggles are usually more complicated than that. Here we are more than a month later, and the problems that Lincecum had at the start of the season are still present. He simply hasn't made progress.
Velocity is part of it, but bad control and an inability to put hitters away are also still plaguing Lincecum. According to Andrew Baggarly of CSN BayArea, Lincecum thinks his composure has been poor as well. He's shown bad body language, and he thinks it's given hitters an excuse to be even more comfortable in the box against him.
The Verdict: Slump
Lincecum won't regain his lost velocity, but I refuse to call him washed up, because diminished velocity is merely one problem among many.
Lincecum will get things figured out eventually. A pitcher with his track record and his stuff is not going to have an ERA of more than 6.00 all season.