Yes, the thumb injury hypothesis seems flawed, because he has thrown some brilliant passes since then. The touchdown to DeSean Jackson against the Buccaneers was beautiful.
At any rate, Matt Stafford himself cleared all of this up yesterday when he said that he just 'missed some throws'.
We don't need to look for a grassy knoll or an Area 51; he just isn't playing well.
It happens.
Also, one of the announcers pointed out during yesterday's broadcast that Stafford has been streaky his whole career. I hadn't thought about it before yesterday, but that is exactly what I would say about Jared Goff.