It's a shoulder problem, and he'll have an MRI. At least he's not a pitcher, but if the team ordered an MRI, it may be fairly serious. As far as I know, Tabata has no history of shoulder issues. Obviously, it'd be a tremendous blow to the Pirates' future if this turned out to be anything bad. Tabata himself did say earlier today that the injury wasn't serious, so let's hope he's right about that.
UPDATE: Tabata has "mild rotator cuff inflammation," but no structural problems. Hopefully, this won't be a big deal.