I'm certain that Mullins, Rutschman, Mateo, Urias and Frazier are all good defenders. I think that Henderson will be also. Santander is fine in RF. Hays, and Mountcastle are the weaker defenders. Stowers I'd expect to be passable but below average if given more run, comparable to Santander/Hays. Ortiz and Cowser would be above average if they get their shots.
Not arguing that it hasn't been pretty shaky to date, but I don't consider team defense a concern moving forward. And aside from getting Cowser and Ortiz into the lineup, there isn't an external fix that should improve things. The guys we have need to play good defense, as almost all of them are capable of doing.