I think there is much uncertainty in Santander's projections going forward. He had a .565 OPS in his first two shortened seasons in Baltimore, then a .773 last year. One story is that he was yanked out of the minors as a Rule 5er, and that sidetracked his development. Plausible, but doesn't necessarily mean he's sure to be an .800+ OPS guy. We need more data to go on, and unfortunately we won't get a whole lot this year.
He ran out of gas in September and had a .363 OPS from September 8th onward until getting hurt and missing the last week of the season.
His OPS through 77 games was .854, his final 16 games dropped his season OPS to .773.
I think Stewart and Mountcastle will likely also get some time at DH with Nunez occasionally sitting against righties and Ruiz occasionally sitting against lefties.
With Mountcastle also being the backup first baseman, there should be plenty of at-bats to go around.