Yup, Bingo. I wouldn't call Marsh mediocre but he still can't play full-time. That said, platoons get you better production from all the players, they are only mediocre when they don't platoon.
I think you keep playing Casty unless or until you have someone better. None of these guys are. Moving Sosa or Turner to OF is not the answer either. Ironically demoting Rojas means Dahl and Merrifield and Pache are less available to play RF too.
And agree with Andy that Dahl already seems to be no more than the AAAA guy he seemed to be after a brief hot start too.
I guess a Dahl/Marsh OF against RHP is better than not having a second LH OF and the continued redundancy of Rojas/Pache but that still leaves Casty playing every day, and far too much Merrifield. I'd rather see Pache play LF I think.
They just went 10-10 over the last 20 in difficult circumstances and still picked up two games on Atlanta. If you think that's a real reflection of what the team is going forward than yeah I guess it's time to panic. Demote your CF. And then, inevitably, give up too much at the deadline for a new OF. Maybe in the end that's a good thing if it's what forces them to move on from Castellanos. Get a new RF and make him the platoon LF with Marsh. But you still need Rojas in that scenario.