Quality back-up catchers on the free agent market
This is the group that, hopefully, the Astros will be picking from this offseason. These are all guys that may not be good enough to be primary catchers under ideal circumstances, but have the experience to pass along useful information to Diaz about how to succeed as a catcher while being able backstops themselves.
The name to keep an eye on here is Victor Caratini. While he had to yield to the arrival of William Contreras in Milwaukee last season, Caratini put up strong defensive and framing numbers while putting up a reasonable .711 OPS at the plate. Caratini has been highly regarded as a back-up catcher for a while now and may be the best of the Astros' options here.
Gary Sanchez hits absolute tanks, but isn't particularly strong in the field and his hit tool is very, very suspect. If the Astros are looking for a high offensive upside backup, Mike Zunino is probably the best option as he has shown to be a decent defender in the past while also possessing the ability to hit the ball out of the park.
Yasmani Grandal's best days are behind him, but he isn't uninteresting as a backup option with Hedges, Barnhart, and Piña also all having experience as quality back-ups with some shortcomings at the plate that have kept them from getting primary catching roles.