Baseball, like all sports, is a game of matchups. Hitters fare better against certain pitchers. I bet that's part of why the rotation adjustment was made. Throw Roark in there and see if the Braves can hit him.
Strasburg and Clippard are the poster childs of this. The Braves crush them for whatever reason, and it's not for nothing.
The Nats are faring better with the bats this year, so things may yet turn around. Simmons being injured should help with that. How long is he out?
