I disagree with that.
Wang's Platoon Splits
Webb's
Carmona's
Lowe's
Between them, Wang, Lowe, Carmona and Webb are, in no particular order, perhaps the best sinkerballers in the league. All of them have significant platoon splits. Some more than others of course (Webb's surprised me by being frairly significant) but it's a hurdle sinkerballers learn to overcome if they're left in the rotation. I do NOT believe that three bad starts made that a different thing for Masterson than for those other 4 starters, especially considering his upside and just how effective that sinker is for him.
JMHO, but I think Theo let the illusion of depth lure him into making the easy call and it cost us in the long run.