We don't know nearly as much about Jarrett Stidham as we'd like.
We don't know if he'll be a good starting quarterback, a bad one, a great one - actually, we don't even know if he's going to be the Patriots starting quarterback. For all we know, the Patriots like one of the kids in this draft so much more than Stidham that we'll never even see No. 4 (his number, had to look it up) under center.
But let's say Stidham is the starter, and let's play the guessing game. Assuming health, the worst-case scenario isn't Stidham being bad; it's Stidham being mediocre.