The conventional wisdom around here is that Goff cannot be evaluated at all. We can't know anything about what we have in him. How then, could we possibly know anything about Mannion?

Never the less, whether or not the "right" QB is on the roster currently isn't the point. Kaepernick wasn't on the 49ers roster when Harbaugh took the job. Brady wasn't on the Patriots roster when Belichik took the job, etc.