Want to know which non-established player is being given a real chance to stick with the team? Count their reps during the preseason games.
The one exception might be those rookies who they don't want anyone else to know about, and who they hope to waive and then put on the PS.

Mannion is the 3rd QB. The goal is to co-prepare Keenum and Goff as starters 1 and 1a.
There is simply not enough game reps to give all 3 an "equal" chance.
And especially so, as Fisher pointed out, when the D allows the opposing O to stay on the field and own the clock. That way our O gets even fewer reps.

There is no doubt in my mind that the plan is that next year Goff will be the starter, Mannion his backup, and Keenum will not be on the Rams.