He was
far more than serviceable in 2017 and even if he never improved from there he'd be well above average. Now he's also not elite, and the climb from top half of the league to top 5 is pretty steep. And he doesn't have to get there this year.
But I do buy into the idea that he's set up to improve not only because he's paired with McVay but because this offseason is about refining his mastery of the offense and evolving it further, not learning it, while continuing work on his mechanics. Big, big difference from a year ago.
McVay made big strides with Cousins in their 2nd season together (86.4 to 101.6 QB rating and a 19.7 point jump in ESPN QBR) and while that sort of jump is highly unlikely for Goff, there's no reason to think he cannot and will not improve in his decision making. His biggest issue a year ago was holding onto the ball a second longer than he should have at times, which to me was a pure by-product of learning the new offense. Get the ball out consistently even a fraction of a second faster and his results could be better (and fumbles fewer).
Their O-line is also critical for him. Their performance was outstanding last year and continuing that will be important. They started investing in their future OL in this Draft and hopefully 2 of the 3 drafted become solid starters for them.