The only pitcher I'm absolutely certain will pitch in the starting rotation this year is Yovani Gallardo. It's also incredibly likely it will feature Chris Narveson (returning from shoulder surgery) and Marco Estrada. Mike Fiers will probably also make the cut. Beyond that, I'm not really sure. Mark Rogers? Wily Peralta? Tyler Thornburg? We'll probably have to wait until Spring Training is nearly over to know for sure.
Gallardo, Narveson, and Estrada are known entities, while the others become increasingly less so as we move down the list. The good news is that with the aforementioned high-powered offense the Brewers shouldn't need a league-leading pitching staff, they merely need a serviceable pitching staff. They need the starters to make it through five to six innings in most of their starts so too heavy a workload doesn't fall on the bullpen. As usual we can occasionally expect Gallardo to pitch deep into games. If the starters can do this the Brewers should be fine, especially if one (or more) of Fiers, Rogers, Peralta, or Thornburg exceeds expectations.