Hoping to understand Alfonso Soriano's hot and cold streaks this year, I turned to the incomparable Fan Graphs to break down his present and historical success against different pitches.  The results suggest that Soriano is losing the skill that made him one of the more feared hitters in the game, but that he might have found a method to compensate for this loss.  Below is a chart showing the percentage of fastballs Soriano has seen each year since 2005, with 2009 broken down per month. It also shows his ranking among hitters seeing the fewest fastballs, his "runs above average"  number on fastballs, (wFastball) and how high he ranks among all hitters, and his overall OPS. (As in, not specific to fastballs)  The most important thing to notice here is his wFB rank.

You probably have noticed a couple of striking trends going on here.  First, Soriano has progressively moved from being one of the most effective hitters in baseball against the fastball to being quite pedestrian. Second, pitchers have not noticed and adapted to this change:  They contiue to avoid throwing fastballs to Soriano as if he were the same hitter he was in 2005.  He's not.

So how do we explain Soriano's April and July, when he hit like the hitter for whom the Cubs offered that premium contract?

Finding  that answer requres looking at Soriano's results swinging at sliders.

It makes sense that if pitchers fear Soriano's ability to hit the fastball, they would throw him an inordinate number of sliders, and indeed the table indicates Soriano consistently sees among the most sliders in the game.  Throughout his career Soriano has proven to be one of the weaker hitters against sliders.

This potentially could be a toxic combination: A fastball hitter who no longer can hit fastballs, and still doesn't see any fastballs to boot.  But then look at the two months this year that Soriano has hit: April and July.  The numbers indicate that Soriano's output isn't due to hitting fastballs with the authority to which he is accustomed; instead he's hitting sliders in a way that he previously did not.  Well.

Of course, other factors might be at work as well, but if so I have yet to find an obvious candidate.  Such marked changes do not appear when looking at Soriano's results against other pitch types.  Neither does plate discipline seem to be the issue. Soriano is both swinging at fewer pitches outside of the strikezone this year, and making more contact when he does swing - both at pitches inside and out of the zone, and it shows no meaningful fluctuation from month to month in 2009.  His BABIP shows severe fluctuations from month to month, but that appears to be driven by corresponding changes in his line-drive percentage, so there's nothing unusual there.  

Assuming that these stats do reveal a truth about the real world (I know, I know), there are several questions to consider going forward.  Most obvious, what is the cause behind Soriano's diminish returns against fastballs?  Is it that he now has a "slider speed bat"?  Is it a matter of approach at the plate, where he has stopped guessing fastball in order to better hit the breaking stuff?  The latter can be fixed, the former, not so much.  Looking at how consistent the downward trend is, my hunch is that it's the former, a slow erosion in the speed of his bat.  If it's the former, and Soriano is losing some quickness in the swing, will we see him move away from his preference for heavy bats?  And will he be able to compensate by consistently hitting sliders with the results found in April and July?  Then, there are questions concerning how pitchers resond.  At what point will pitchers adjust to the fact that Soriano does not hit fastballs with the authority he once did, and adjust their pitch selection? There may be a lot riding on these questions.

Update: I was unaware of it as I wrote, but less than a month ago R.J. Anderson at Fan Graphs wrote about how few fastballs Soriano sees.  He doesn't mention either Soriano's historical trend of diminishing results on the fastball, or the weird fluctuations this year on the sliders; instead he reaffirms the notion that Soriano is a good fastball hitter and the problem is in the dearth of fastballs he sees. I think he may be wrong there, but I should at least try to be a good scholar and cite the work.