The big question is what is the pitching staff going to look like *if* Sale and Paxton come back, are efficient starters, and everyone stays healthy.
I may be overthinking things and getting too abstract, but I'm not thrilled about using 42-year old Rich Hill as a reliever, he can stay as the 5th starter/opener. He's 42, he needs to be warmed up properly on a regular schedule, not told to be on call to get up and start throwing in the 4th inning.
Houck's been a very good starter this year, but he's really well postured to go into the pen as a right handed Andrew Miller type to give you 3 innings in high leverage middle-innings, I'd have no hesitation making that move.
Wacha has outpitched Pivetta so far, and Pivetta actually was lights out from the pen in October because he's got a plus curve and can crank a good fastball when he knows he's working 1 or 2 innings max rather than having to conserve energy for a 5-inning start. The big question is whether the organization's goals are for him to continue to develop as a starter and whether he would accept what looks like a demotion by going to the pen. The Red Sox definitely have more invested in Pivetta and Houck as long term pieces than they do in Wacha.
It's probably all moot, since the chances of all of these pitchers staying healthy is unlikely, plus with the schedule being compressed thanks to the lockout, there will probably be stretches where we need to roll a 6-man rotation at the least.