Incapable in what way?
If Hamilton and Rengifo somehow both got injured in pregame warmups on Friday and Murphy decided this was his best option for a game, he would get through it and the Brewers would still have a chance to win the game. It would not be a complete clown show every time the ball was hit to the left side of the infield.
He doesn’t play the position because he doesn’t get any regular reps there. If Murphy for whatever reason wanted to try it out, he’d start getting regular reps there, and they certainly wouldn’t try it in the middle of a season,
I have no idea if he’d be capable or not on a passable basis, but I’m certainly not going to jump to saying he “can’t”. These are professional athletes, even the worst ones are among the top .001% at what they do and far more athletic than we could ever dream of,
No, he would probably not be good there. He's not particularly great at 1B.