Casas hasn't been that bad lately. In the month of May, he is hitting. .250/.355/.442 That's a .797 OPS. He will also continue to just be 23 all year and conceivably is learning to adjust to MLB pitching. He should get better. Duran has cooled down substantially as well, but he still looks a shade better than he did during his first couple stints in Boston.
If Ohtani was a RHB, he'd be the perfect fit for this team next year. A Right-handed bat and an ACE. But there will be other options for pitchers such as Urias, Nola, Yamamoto, and Giolito will headline a pretty nice class of pitchers headed by Ohtani.
The position player side of things isn't exactly exciting, especially considering we would like a RHB. The best options for a RHB might be Teoscar Hernandez, and Matt CHapman, but we don't need a 3B or a corner outfielder right now, unless you consider trading Devers or Verdugo. The later seems more likely than the other but not likely.
I think the team will either have to trade for a RHB, is there a MLB team out there that is the right-handed hitting equivalent of Boston? or we will have to fill in the roster with a few platoon bats that can really mash LHP, like we have this year with Refsynyder. Of course, if Trevor Story can ever get healthy, he could end up being a big RHB in the lineup.