The Red Sox can't get Lefties out. Masterson is no better than Timlin, Hansen, and Lopez. In fact Masterson might be the worst out of that bunch. I don't see what he is adding. They need someone who can get lefties out.
Actually, Delcarmen does it well enough, presumably with the change. Timlin doesn't really get anyone out, and Hansen is a FB/Slider guy so that obviously wont work against LHH. Lopez has bad splits this year.
Okajima has been bad recently, but he was also effective against LHH last year (though he was effective against everyone).
Pauley and Hansack are not great options and Colon is injured. Right now Masterson is giving you solid starts each time he goes out there. Why give up a solid starter for a ROOGY? I would rather Buchholz in the bullpen.
I don't think he will be used explicitly as a ROOGY, but I do think he will primarily face RHH. What I'm trying to say is, I don't think you'll see him face one batter and leave, but that Tito will be, as always, careful with the matchups.
A number of other people have noted that with the exception of his ERA, Masterson's motto as a starter could be, "I also
like to live dangerously". He allows a lot of baserunners and has gotten great luck with his sinker. His dominance of RHH has really carried him this far. I see no reason why you wouldn't want to substitute this for a guy who you think is ML ready and put your RHH dominance in the pen where it can really be exploited.