I agree Mookie should be playing just about everyday. Probably primarily as a RF initially, possibly with that altered as the season goes on depending on Victorino's health and Castillo's performance.
Having said that, is there any reason to see a reasonable likelihood of Ramirez, Castillo, Victorino, Ortiz and Pedroia all playing 152 games this season? That seems extremely unlikely to me (and borderline impossible with Victorino).
Even if Betts is just the primary backup instead of a starter (again, not my preference), there is a very good chance he will get significant playing time. The one caveat is that of course some of the games missed from those 5 might be in the same game, in which case obviously Betts can only fill one spot. But I'd still keep him on the roster and play him 4 to 5 times a week and give each of those guys a little extra time off until an inevitable injury occurs.
FWIW, in 2014 Bogaerts lead the team in games played with 144. In 2013, Pedroia played 160, and #2 on the team was Mike Napoli with 139. I think a pretty optimistic projection even without major injuries would be an average of 140 games for Ortiz/Pedroia/Castillo/Ramirez and 120 games for Victorino. That leaves 130 games of PT. Again, some of those missed games will overlap (and other guys need playing time too), but I'm not sure projecting a 152 game season for all 5 of those guys is particularly useful when trying to assess how the season is likely to unfold.