He turns 28 next week. Forget Casas for a moment and recognize that Turner also plays 1b. He should be the DH, but right now the Sox have five pretty good hitting outfielders in Duvall, Duran, Dugo, Yoshida, and even Refsnyder.
Of those, easily the worst fielder is Yoshida, so he is kind of competing with Turner for the DH slot--assuming the Sox want Casas to play 1b.
So my point--actually, the point raised by others--is that the Sox could probably do very nicely without Casas or Dalbec. Turner could play 1b every day and Yoshida could DH, which would improve the Sox outfield defense with Duvall, Refsnyder, Duran, and Dugo available.
Nevertheless, Casas is getting to play, I think, because he's 23 (24 in January) and appears to be able to hit MLB pitching.
And don't forget that Dalbec has had his chances. He played in 133 games in 2021 and 117 last year, so the Sox have had really good looks at him at this level. Last year his OPS was .652 and his WAR -0.7.