I'm not going to create hardline answers to things I couldn't possible know.
But with the Cubs electing 12 pitchers over 13, guys like Rea and Civale are likely your "designated losers" to eat a few innings if a SP is struggling. There is no tomorrow if we blow games and there is no shaking things loose. If Taillon, Shota, or Boyd are struggling in the 3rd inning, regardless of how good they can be, you pull them. And then you turn to a Civale or a Rea and you say "I need two or three innings, go stop them while we get back into it".
It's far better than using Theilbar, or Keller, or Kittredge in the fifth when you'd ideally want them in the final third. Or in a 4+ run loss, to not use them at all and get them tomorrow.
Thus the "designated loser". They come in when you're losing to eat valuable innings and perform good enough to keep the game within reach (if possible) for your offense to scratch some across. I think in these oddball three game sets (that function as regular season sets despite them being in the playoffs), they're incredibly important. With no off days, you don't want Keller going three days in a row, for example, if you don't have to.