I'm very content to have Lannan, Bergmann, and Redding atop our rotation. Hill's operating at about 75% capacity right now, unfortunately. Perez is shaky, I'm worried his recent trend is going to continue. But Sens, I see no reason why you're so skeptical about Lannan or Redding. Both have good enough stuff, excellent control, and demonstrate the ability to focus when they get in trouble.
Bergmann's been inconsistent historically, but as evidenced by his last two outings, he's got excellent stuff and can be dominant for long stretches of time. I can't wait to see him pitch again.
But getting back to the point of the thread, we need to have somebody identified soon as a rotation replacement. Hill's already showing issues, and history shows we are certain to lose guys during the season. If Mock, Clippard, or Balester were lights out, nobody would be pushing Zimmermann or Van Allen, but since they're not quite lights out, it's a fair question.
EDIT: I fixed the spelling of Zimmermann in the thread title. Zimmerman has already had his fast track to the majors, it's Zimmermann we are now concerned with.