This is, by far, the best source I've found for injury information from past seasons. It only goes back to the early part of the decade (so far), but even with that limitation, I'm already wondering how I went without it. For all active players, it's got dates and duration of DL stays, descriptions of injuries, and context on related injuries that same season. Kill a few minutes this morning and give it a look.
The baseball medical blog it's attached to is worth a look as well. The writer, Corey Dawkins, is the head athletic trainer at Emerson College, and he seems pretty coherent and intelligible for someone with such a high degree of specialized knowledge.