For all his offensive wizardry their back up QB situation is a mess. Not one of the 3 should be on the team. I really hope they can make a late trade (RG3) or grab a guy someone else is trying to sneak through waivers (like the Saints did with Hill after the Packers tried to hide him). You could carry 3 for a couple weeks if Obo goes on PUP and then cut Mannion loose after new guy is acclimated. Of course then you're on the hook for Mannions full contract.
Best would be to add new guy - cut Mannion. Then grab Mannion as a #2 down the road if, god forbid, something happens to Goff. I really can't see Mannion getting signed by anyone else at this point. Just tell him to keep his phone on and stay in shape.