Unless he's a rookie. Even Manning sucked as a rookie.
I would say you need a great o-line if you are going to start a rookie, or you could ruin him.
Since I don't see any great QBs available via FA, and drafting a QB is a crapshoot (anyone who thinks otherwise might want to think again), our safest bet is to somehow make the o-line at least adequate. It really should be the priority, but I don't see it yet.