If Goff goes down and Mannion becomes the starter, you can kiss the Rams playoff chances goodbye. He's slower a foot, has a slower release and isn't nearly as accurate as Goff. I've always felt like a backup QB has an advantage in a short term starting role if his game is different than the starter. It takes awhile for defenses to adjust to the "style" of the new QB. There is nothing better that Mannion has compared to Goff, it's only worse in every aspect.
I hope the Rams pickup a viable backup to Goff cheap in the next week or 2. The Rams can keep Allen and fill in that backup role if needed until the newly acquired upgraded QB is ready to go if in fact Goff is out anytime during the season.