Rams could have cut Mannion in the offseason, and signed RG3. Salary cap hits are about the same. Guessing Allen will be the backup if Mannion has another game or 2 similar to last night. RG3 would already be on the roster if McVay wanted him over Mannion.
Couple of the returning backups, Mannion & Hill in particular, struggled. Do not see the roster spots in jeopardy, yet. However a another poor performance or 2 will certainly change things quickly. The NFL does stand for Not for Long, when backups perform poorly when given a chance to play,