The problem with sending Joseph down is how would he play? At DH, I guess. I don't believe they would bring up Hoskins (the fact that they have added Leiter and Kelly to the 40 suggests that too, even if they are disposable). They could keep Kelly and play Nava and Stassi (might help Stassi since he is not doing anything with limited action).
But having a sophomore slump is normal, and something you might have to play through so you can adjust to the things you are failing to adjust to, which won't exist in AAA (and if it's not a slump, then that's that). Granted,there might be value in just letting him tee off in the IL for a few weeks to find his timing. What's really killing him is he's not hitting LHPs, nor is he getting to face enough of them to find his way out of that, because he's never going to hit RHPs as well, long-term. But hey, they also don't have to trade a player this off-season if Joseph is only a platoon guy.
It's looking like Saunders might be the guy to lose playing time to Altherr, at some point R/L doesn't matter if one guy is hitting and the other guy isn't. But if Kendrick is slow to feel 100% or struggles that will make it easy too.