Maton can play the OF if he has to. Camargo probably could too. Things are just still messed up because Harper can't play the field.
I don't think they've ever actually carried six OFs for long... it's just that the normal group of five has Harper in RF and one of Castellanos or Schwarber as the second bench outfielder (the opening day four-man bench was Bohm, Camargo, Herrera and Stubbs. Still only one OF).
But it is currently a three-man bench with zero infielders (except Vierling). When they go back to four Maton probably still makes more sense than Moniak. There's just no reason to have both Moniak and Herrera, and with Segura gone the extra middle IF is more useful. Vierling adds something there of course but he still can't play SS, and Maton is probably better there than Camargo (and Didi too but that's another story).
