Stafford is McVay's QB of choice. Since it is clear as day that he felt that Goff was the problem in the offense, then the cost of jettisoning him and acquiring Stafford falls directly on McVay.
We can all say that Goff was McVay's choice as well, but that's only partly so. He didn't draft or acquire Goff; he only thought that Goff was on a trajectory that he could succeed with. What were the Rams to do? I don't recall anyone thinking that resigning Goff was anything but a no-brainer. But the Stafford deal is entirely different. It's 100% on McVay.
My prediction is that we'll see the offense take off again with Stafford. Not because Stafford is necessarily the better QB (although I think it is generally agreed that he is), but because McVay will have confidence in him. Because McVay knows him and likes him and must believe he can succeed with him.
McVay knows that this is his moment of truth. His tenure with the Rams will be long or shortened depending on how it goes with Stafford. I have great confidence in McVay. I think he took this year's team even farther than it had a right to go.