Ivan Herrera or Tres Barrera
The backup catcher issue feels like nitpicking, but it's obvious that Andrew Knizner has not performed well and that his days in a Cardinals uniform may be numbered. This is where Herrera and Barrera come into play.
The issue with Knizner has been his hitting. The Cardinals' backup catcher is hitting just .143 on the season and has never been able to get it going at the plate. Barrera had a strong spring and is producing quite well at Memphis, hitting .263 with three home runs. Herrera has also performed quite well, hitting .255 with two home runs. Herrera is also a solid defender behind home plate.
With Knizner struggling, the Cardinals could definitely use a reinforcement behind Willson Contreras.