IMO, the need to acquire a lefty is pretty clear. Using .725 OPS as the line that separates adequate from inadequate, here is how things shake out:
Against LHP the guys on the adequate list (limiting to the main 13 position players so far): Frazier 1.049, Votto .998, Ludwick .952, Hanigan .878, Cozart .872, Stubbs .822, Cairo .818, Bruce .794, Heisey .780, Mesoraco .736 and Valdez .727. The inadequate list would be only Phillips .607 and Rolen .486. So against LHP, the Reds keep coming at you over and over with 11 of the 13 guys threats and the 2 that aren't have traditionally killed LHP.
OTOH, against RHP the adequate list is limited to Votto 1.214, Phillips .873, Bruce .854 and Frazier .848. Two of those are a bit iffy going forward. Frazier has a pretty big weakness on the ball running or breaking away to the outside part of the plate (i.e. a breaking ball from a RHP) and Phillips has traditionally struggled against RHP. I think the league will adjust to Frazier and just don't want to count too heavily on Phillips continuing to buck his career trends. The line-up is pretty thin against RHP.
The inadequate guys include Hanigan .693, Cozart .685, Mesoraco .675, Heisey .653, Ludwick .634, Stubbs .619, Rolen .566, Valdez .405 and Cairo .343.
Since they face RHP about two-thirds of the time, I think they need to even this out. It means adding a guy who hits against RHP and removing one of the redundant lefty killers. My vote is Ludwick since he's the most likely to have his PAs taken by Frazier when Rolen comes back.