Casas is 23, 6'4" and 255 freaking lbs, which to me means he is not serious about being a professional athlete.
Because everyone else stunk at 1b last year and it looked as though he could hit, he was basically handed the job this year. And he hasn't done much with the opportunity because he's 1/3 into the season and has the lowest WAR on the team, -0.7. Nevertheless, it's clear Cora has instructions to give Casas every opportunity to show he can play--basically, hit--at this level.
But two things could limit Casas's ownership of 1b this year if he doesn't improve his hitting.
First is whether Bloom and especially John Henry want the Sox to get a wild card berth. If they do and Casas continues to have a negative WAR, he could sit more or go back to Worcester.
Second is the return of Duvall, who will be the 4th outfielder along with Yoshida, Duran, and Dugo. Little doubt in my mind Duvall will play. If he's in the outfield, either Duran--who is hitting and playing better than Casas--sits or, more likely, Yoshida takes over at DH. If that happens, Cora ain't going to bench Turner so that Casas can play 1B.