H/t to hutch for being on top of the story.
I exchanged emails with Will Carroll, the injury guru of Baseball Prospectus. He wrote a post over at LoHud back in January basically saying that we can't use minor league innings to forecast the stress of a major league season.
So I asked him how innings from the pen would forecast. Here's his reply:
[Innings in the pen and in the rotation are] very different, but I don't have a "factor" for an accurate translation. No one's done the study yet, but there's more behind moving from the pen to the rotation (Adam Wainwright) than there is the minors, oddly enough.
I'm not entirely sure what to make of this information. I confirms my own suspicions, and it makes me think the Yanks are really handling the Joba-to-the-pen situation backwards.