If he goes to XST on Opening Day, what is his status as far as the 25 man goes? I think that's the question that needs to be addressed.
If he's on the 25 man, and just not with the team, and the Dodgers are playing a man short for however long, I think you'd have to leave him on your roster and take the 0...much as the Dodgers would be doing.
The other options are the DL or demotion to the minors, and those are self-explanatory under our rules.