No, see, that's where you are wrong. In this case, Dani Alves vs. Montoya, you were unable to make the difference between a top top player, who hit a rough patch (along with several other players), and an average player who's form was always linear (or very little). Montoya is that type of player, who's game is so simple, that his mistakes are never catastrophic because he isn't
I don't doubt players very quickly myself. Didn't doubt Alves, didn't doubt Pique, didn't doubt Alexis, not even when they played well under their value. Didn't doubt Cesc either, although he's not proving me right these days. Sometimes you have great players who perform under their value, sometimes you have players of lower value who just play to the best of their limited potential. I only doubt players that I think are not good enough. Not based on form.