There's how the roster looks to start the season vs. how it looks in June-July when we have a better sense of how good the team is, vs. how it looks in August-Oct for a hypothetical stretch run. Sort of like Billy Beane's old saw, "You use the first third to see what you have, the second third to get what you need, and then ride the last third to the playoffs." Or something like that.
So, for the outfield, it seems likely that, as the author says, they start with Trout and Ward, with Moniak on the strong side of a platoon with Hicks and Adell. Trout and Ward can get DH starts, so after 50 games we might see games started like so: Trout 45, Ward 45, Moniak 35, Hicks 20, Adell 20 (150 outfield + 15 DH starts). Adjust for injury and performance. By June, you've got a third of the season in the bank and have a better sense of things, and can re-adjust.
Adell pretty much has to start on the big league club, unless someone offers a similar "tarnished former top prospect with untapped upside" in return, which is very unlikely. You just can't trade him for peanuts, not at 25 years old (can't believe he's going to be 25 in a few weeks).
That said, Hicks and Marisniak, despite strong springs, are unlikely to be anything more than the 4th outfield types they've been for most of their careers. Meaning, their upside is limited - and quite a bit below Adell's (and probably Moniak's) - so you really don't want to completely block Adell from getting his chances. The scenario in which one of them retains a job over Adell for the entire season is basically if all of the above happens: Trout and Ward are healthy, Moniak is playing well, AND the Angels are competing. Then you want a high floor 4th outfielder. But that's a lot of "ifs." Chances are someone gets hurt and/or disappoints, or the Angels simply aren't that good. Then Adell's sporadic playing time expands come June.
And of course Adell could theoretically force the issue by earning more playing time. But unfortunately for him (and the Angels), he probably needs a solid 400+ PA of consistent play to un-tap his potential. Meaning, it is hard to see him mashing in a sporadic 100 PA in the first couple months, forcing a full-time gig. But I hope to be proven wrong.