But obviously yes we're fans. All we know is what we see on the field. The coaches know these guys far better, from practices on to film and mental type sessions. If we're going to decide that a player doesn't know the scheme that well it requires him making mistakes in a way where we think on the field that he's not up to speed mentally. But we don't know that.
Funny thing is fans are often right about things too. So to some extent there are advantages to judging a player ONLY on what he does on the field. Blythe for example the coaches know him and care about him but the fans see him turnstiling with defenders and are done with him. Well the fans were right there.
Where the fans often make mistakes is when we assume. Like with Stafford. Most of us hadn't watched much of the Lions but we all had opinions on Stafford. For me it took watching his games after the trade where I realized that I had him wrong. Media of course don't want to watch film, they're the worst offenders IMO outside of a few here and there who are dutiful in backing their opinions by film.
But in the end the fun is talking about your team. Not being correct. Who the F cares if you were right/wrong in your guess about what the team will do or who will play well. Not like there's someone keeping score somewhere and it means something. We're fans talking about our team. It's supposed to be enjoyable in and of itself, at least the way I see it.