Eric Seidman at Fangraphs uses Dan Szymborski's new in-season ZiPS leader boards to find the worst starters that will likely see a significant number of starts the rest of the year:
Adam Eaton, Orioles: 19 GS, 99 IP, 1.53 K/BB, 6.09 ERA
Franklin Morales, Rockies: 19 GS, 97 IP, 0.89 K/BB, 6.03 ERA
Rick Porcello, Tigers: 20 GS, 95 IP, 1.19 K/BB, 5.97 ERA
Dustin Moseley, Angels: 20 GS, 111 IP, 1.71 K/BB, 5.92 ERA
Including performances so far, that would make this seasons' worst overall starters look like:
Adam Eaton, Orioles: 25 GS, 130.1 IP, 1.55 K/BB, 6.35 ERA
Carlos Silva, Mariners: 27 GS, 149.2 IP, 1.94 K/BB, 6.31 ERA
Sidney Ponson, Royals: 21 GS, 125.0 IP, 1.28 K/BB, 5.83 ERA
Franklin Morales, Rockies: 21 GS, 101.0 IP, 1.01 K/BB, 5.83 ERA
Now, what issues might we have here? One, ZiPS isn't really paying much attention to projecting playing time, like most projection systems. Two, as with most (all?) projection systems out there, ZiPS doesn't know about injuries, new pitches, or other scouting-style data.
With the prevalence of pitch f/x these days, I wouldn't be surprised if we see a projection system for pitchers that utilizes that data to some extent. Changes in fastball velocity, control, pitch movement, and pitch selection should be able to help tell us a pitcher's story objectively.