Aside from a big injury epidemic, which can happen to anyone, the only way they go 4-12 is if both Goff and Gurley turn out to be subpar this year.
Which by extension means Mannion does too.
They already went 4-12 last year, and that was after a move, with a rookie Goff, Gurley playing subpar, a line not as good as this one, receivers not as good as this set, with a series of key injuries on defense, and against a tougher schedule (last year's schedule was 3rd toughest; this year's is 17th).
....
