The outfielder had some back issues in September as well.
There's a lot of unknowns here: Is this the same injury, or a different issue? Upper back or lower back?
I have a certain amount of personal experience with back problems, and I am confident of this: If Arcia's having problems in his lower back, he also has -- or will have -- problems with his hamstrings.
At this point, I would think it wise for the Twins to get Arcia back to the States and find out what's going on with his core. That would be prudent, not an overreaction -- not after nearly two weeks and a promised return that didn't happen.