Yeah, I lean more toward keeping Mannion for 2018 and then signing a veteran backup in 2019. If McVay and LaFleur can turn Goff into a quality starter why wouldn't they also be able to turn Mannion into a quality backup? None of us really knows what McVay really thinks of Mannion though I think if they didn't like him they'd have made a move before the season.

Mannion will make an $878k salary next year with a $1.03M cap number. Pretty modest and any experienced backup-type signing would be 2-3X that.

For 2018 I'd go with Mannion and continuing to get him more comfortable in the system. He's only thrown 14 regular season passes but his career preseason numbers are not bad:
119/193 (61.7%), 1,115 yards, 7 TDs, 2 INTs, 85.3 QB rating