What his problem, at least according to pitch f/x was, was that he started throwing a two seam fastball and it killed him.
Go back to his first year and he got great results with his four seam fastball... add in the two-seamer, and bad things start to happen.
2008:
62.9% Four Seam Fastball
18% Curveball
10.5% Changeup
8.4% Slider
0.1% Intentional Walks
0.1% Pitch Outs
In 2008, Fangraphs had his fastball at 10.2 runs above average... from there, things go down the drain.
2009:
54.5% Four seam fastball
12.4% Slider
11.7% Curveball
11.5% Two seam fastball
9.3% Changeup
0.5% Intentional Walk
-4.8 runs below average on his fastball. Coincidence that his fastball starts to go south when he adds a two-seamer?
2010:
34.8% Two seam fastball
31.9% Four seam fastball
15.5% Changeup
12.1% Curveball
5% Slider
0.6% Intentional Walk
0.2% Pitch Out
-6.8 runs below average on his fastball. Two seam fastball is what he was featuring the most this year and he flat out sucked. I think what's been killing John Lannan is the fallacy that he was a sinker ball pitch... when in fact, he got by on his four seam fastball... all this time.