Start in the pen, move to the rotation.
Easy.
But when?
Back in 2003, Scot Shields spent his first full season in the bigs. He compiled a 1.19 WHIP over 148 innings. Aside from two spot starts the last week of April and the first week of May, he spent most of the season in the pen. He relieved in 31 games from April until July, only working back to back days twice but often working multiple innings. That racked up 78IP as a starter, 69 in relief.
Also in 2003, Johan Santana started the season in the pen. He had three spot starts before moving to the rotation full time in July. He'd finish the year with a 1.10 WHIP in 158 innings- 48 as a reliever and 110 as a starter.
I'm in favor of the Santana approach (a more even split in time so Joba throws more innings as a starter than as a reliever. I'm tempted to propose that Joba and Phil switch roles at the All-Star break, but I'm sure that injury concerns will muddle the picture at some point; on the other hand, I hope that the AAA pitchers (Horne, Melancon, Sanchez) put pressure on Cash to find them a roster spot.