With a captip to MLBTR, the Astros were after Twins' catching prospect Drew Butera, according to Peter Gammons:
Astros first choice for C to replace Castro was Twins' Drew Butera, but Twins won't trade him.
If Gammons is right, and there's no reason to think he wouldn't know something like this, then that kind of goes against Castro's replacement being a short-term solution.