So, without wishing to speculate, I'm guessing Robin Finck will move back to guitar, which would free up JMJ to return on bass?
Shame given he's one of Trent's favourite musicians and one of the reasons NIN are back but if it's not working then it's not working. It is quite close to the tour but then I don't think that'll be a major issue so long as he's replaced well.
The rumor is that Adrian found NIN to be too physically exhausting. If you've ever seen Adrian Belew perform live he spends most of his time playing while sitting down, that really doesn't fit into the energetic, aggressive live show NIN is known for.