Ryan Howard saw his home run production and slugging percentage drop in 2010. His ability to slam off-speed pitches declined the most:
The visual representation of the three off-speed pitches makes the point clearly:
Ryan got full power against these pitches in every part of the zone.
In 2010, his coverage was nearly limited to the lower middle of the plate, where he still hit the curve decently. I wonder if it's a vision issue? His slugging against the fastball indicates he can still hit that pitch, and he can pick up a curve ball, which takes a very different path than a change or a slider. For some reason, the latter two pitches might look like fastballs to Ryan now.