I think the issue is that Fox is not able to play as a right tackle.
Obviously the most immediate option right now would be to play Fox at LT and James back at RT.
But this shakes the line up again and I suspect they really want to give James as many LT reps as possible since he is most likely our LT day one next year.
So they were hoping that DT would be serviceable but unnfortuntealy he has not been.
Also flipping tackles with James going back to RT mid game may not have been wise.
https://www.youtube.com/watch?v=uvOoR3giRlo