My guy is whoever is under center for the Rams.
As for Mannion, to be honest, I think if he did play this season it would almost be unfair to him. He's had limited, if any snaps this season. 1 week of increased snaps wont make up for it.
So if Goff cant go, which I don't understand how its been 5 days and we still don't even know if he sustained a concussion, it might be "fun' to see Mannion, but I don't think from a timing perspective or a familiar with the players perspective, that he is in any type of situation that will allow him to play well