Not only are Masterson and Clay not guaranteed a spot, I don't think either of them have a shot at being in the rotation to start the year, barring injury.
Looking at the schedule, the Sox won't need a fifth starter until, I believe, April 12th. That gives Penny an extra week to get healthy, and I can't see them sending him to the bullpen if he's ready at that point. That means Masterson in the pen, Clay down on the farm.
Masterson would be a useful player for your rations if you're punting saves in H2H, while Buccholz has more upside, especially in light of his recent performance. I consider Masterson ownable in deep 10 and 12 team mixed, with Buccholz being more of a "monitor closely guy" unless the league is extremely deep or a keeper format. Right now, my gut says Masterson gets the call if the team needs a spot start, whereas Clay would be the guy if someone went down long term. That could change if he shreds the bats in AAA, though.