I think most of those guys are the depth.
I get the impression Breslow isnt wild about Yoshida, but Yoshida did hit well down the stretch and in the postseason. Casas is the higher ceiling hitter, but not sure when he comes back. But when he does, DH is his best position, especially if Yoshida hasn’t run away with it.
I doubt they trade away enough outfielders to get Campbell a starting job. Hes likely Worcester-bound for April if he isn’t traded himself.
Mayer is a starter. Either at 2b or 3b (although SS really makes the most sense). His role depends on what the Sox acquisitions are.
Romy is a specialty role player. Dont let my “Hes a better option than Arenado” viewpoint convince you I think Romy should start. My point was - Arenado shouldn’t, and argued he was a worse option than the Sox have on the bench. Romy likely settles in platooning with Mayer or Casas/Yoshida. (Although the reality is Casas shouldn’t be platooned. But, you know, Cora.)