Can I repair my shower valve without tearing into the wall?

I have a shower valve where a lever adjusts the temperature and in front of it it a control knob that turns the shower on and off. The center metal portion got stripped and we've been using vise grips to turn it on and off. Replacing the knob won't work; it's too stripped now. Question is, can I somehow take it apart and replace what needs to be replaced in the valve itself without tearing into the wall and installing a new valve? Thanks!