Like I mentioned before, I'm no doctor, and I'm not pretending to be one here, so take this with a grain of salt and remember, Wikipedia should not be your source for medical information.
With that said, based on the symptoms described, which is numbness and tingling in the ring finger and pinky, it is likely that there's involvement of the ulnar nerve. If the pain/discomfort is limited to his hand only, then I would guess that the lesion or involvement to be distal, somewhere in his wrist, or better yet, pretty damn close to the fingers.
Had there been other symptoms, for example, pain in the forearm, which very well could be, I would guess there's an issue further up the arm, maybe around the forearm flexors, or around the elbow.
While I may be jumping the gun here, I am hoping that due to the inactivity and recent stint in the DL, that Voltron simply didn't warm up enough and possibly tweaked his wrist. As far as throwing a ball is concerned, flexors are far important than extensors, and the ulnar nerve makes up one of the two nerves involved in all things "flexors".
p.s. the wikipedia entry on ulnar nerve is actually pretty decent.