Jackson Holliday maybe too? 208 PAs last year (Shaw is currently at 200) with a 62 wRC. This year he's at 104 wRC and underperforming his xwOBA by 24 points.
To be fair, there are probably examples that go the other way. It's baseball, thousands have failed, thousands have succeeded. But he stands a reasonable chance of improving, and his baseline right now isn't fatal for the team success.
Colin Rea, on the other hand, probably isn't going to suddenly improve. Horton and Brown and Boyd aren't going to magically generate a ton of stamina and long term arm strength that will make you comfortable about leaning on them for the next 4 months. I can't see a bullpen guy suddenly turning into a 12 K/9 monster. Sort those problems out first and see where you're at in a few weeks on the (very good) offensive side of things.