Do you understand the difference between knowing where everyone is supposed to be and Gus not being as accurate as you would like him to be?
So on that given play Welker was where he was supposed to be, but the completion didnt happen, ok move on.
As I stated, once Saban can look at the film and consistently say, yup everyone on offense is doing their individual job on every play, but the QB still is not accurate enough then its time to make a change at QB....until then, the issue is bigger than one play that you believe should be completed.