The original plan was Devers-3b, Breggie-2b, Casas-1b
Devers had hurt his shoulder year prior, and they were already a little concerned about its healing, and they liked that Breggie could also play 3b and even 1b and even of, but the plan was to keep him at 2b.
Devers shoulder continued to heal alarmingly slow, and so the plan was to start the season with Devers at DH while he recovered. Breggie slid over to 3b. Eventually, for no reason, things got escalated and wires got crossed and mountains were made about molehills, that all happened prior to Casas injury, so 1b wasnt considered. That was jsut people here oft repeating their preferred defensive alignments.
But as soon as it kind of settled down and Devers accepted that his shoulder looked scary and its a when not an if he gets severely hurt 3b playing defense before the next offseason, and because breggie, it would make sense for devers to dh rest of season.
This is when Casas got hurt
Then they asked him to play 1b as if it would be the magical solution, but he saw himself as a 3b playing dh until his shoulder got ready. He never lost the job because of performance. That was never communicated to him. That was just assumptions from people here who thought it made too much sense to not do.