This is more a Rizzo issue than a Williams one.
This roster is HORRIBLY constructed, as it seems to always be under Rizzo. Span is a left handed bat. His backup (when healthy) is a left handed bat. The 4th OF who would play CF next, McLouth is also a left handed bat. That's too many left handed bats stacked up at the same position especially when they're not good left handed as is the case with Span and McLouth.
When you look at the rosters that Rizzo compiles, you always have to wonder if, while putting it together, did he think big picture, at all.