A quick look at lefty / righty splits for the Cubs who have had > 100 PAs against lefties shows that Happ is the only hitter who does mediocre to poor v lefties and hits righties very well. Rizzo is a complete stud, so I'm not sure you throw a guy like Perez against him rather than Kintzler or Madson. Even Happ has an average wRC+ v. lefties (he crushes righties), so he's doing OK. Zobrist and Heyward are below average against both lefties and righties.
Simplest reason for going with Romero is I guess he's a bit better than Blanton. That's all.
