Will Carroll gives his initial take on Joba today over at BP:

The early word is that his muscle tightened up due to a combination of the blistering heat (I sat in that heat the night before, and yeah, it’s a huge factor) and fatigue. It’s the latter that’s most worrisome given Chamberlain’s switch of roles. While he built up his arm strength, no one has any idea how the fatigue will be affected by the shape of his season. He’s headed back to New York for tests, so we’ll know more soon, but early reports and sources tell me that things look relatively positive, based both on the pain’s location, and on his reaction. Expect the Yanks to be ultra-conservative, and for Chamberlain to miss a start at least.

Nuthin to do but wait. Must be a fun time for Brian Cashman, huh?