Welcome back!
I, for one, am still dreading this matchup. Ignoring the QB question for the moment, the Saints are an extremely talented team. Especially on defense. That's a team the looks very similar to Seattle's from their heyday in terms of effectiveness. It's a team very much built to not have to rely on Star Wars numbers from their QB in order to win.
I think any team that kept on Teddy (including Seattle) as a backup would reliably expect him to achieve at least a .500 record in relief. Perhaps even more. I would think having to roll him out for the next 6-7 games would ultimately cost no more than a game or two less than if Brees was healthy.
I like Seattle's chances. Especially at home. While I think Teddy will be ok for you, I expect he'll have a wide variance in productivity between road and home games. The best way to rattle a backup/league average passer is to effectively pass rush/cover them. Both of these have aspects have been troublesome for us. We played a horrible OL and a very good OL (Cincy and Pitt). We allowed Andy Dalton to set career highs in passing behind the worst OL in the league. One would think we'd consider it paramount that we don't allow that to happen. But Pete is oddly dogmatic in his defensive approach. Teddy could similarly set career highs this week if New Orleans game plans a extra heavy dink and dunk passing attack. Seattle is virtually uninterested in defending the 1-7 yard box. If you game plan for 2-5 yard pass routes, you'll complete them at a better than 70% rate.
Seattle is a notoriously slow starting team. But weeks 3 through 8 over the last several years has seen this team really hit it's stride. We were fortunate that the schedule was kind to us in weeks one and two and our record is similarly blessed. I think Seattle is just getting warmed up -- but the Saints D has a way of really cooling off opponents.
It's not a gimme. Seattle defense has to make strides in these next few weeks. I'd like to think we get aggressive and play WRs tight and dare Teddy to beat us on low percentage deep throws. But I'm not expecting that to happen. Pete's history against bad QBs is to allow the underneath and let the other QB commit unforced errors and lose the game.
I think Teddy is a decent QB, and one that plays at a good to very good level if allowed to remain comfortable. With little pass rush to force him, and an expected vacant underneath zone -- I think Teddy could settle down quickly and find a great rhythm. Even if getting TDs becomes difficult.
Seattle's MO is to keep the game tight. Then it comes down to clutch QB play late. Wilson is perhaps the most clutch QB in the game. I can't recall, but if memory serves, he's second only to Stafford in 4th qtr comeback wins. The main difference being that Russell has a lot more stay ahead wins where Stafford -- not so much.