What I see when I watch the play is that Welker was expecting the ball to be on his inside shoulder but went to his outside shoulder. Why? I don't know and he never says, but it's clear from his positioning that he's anticipating the ball on his inside shoulder. He reacts to the path of the ball, but his reaction is too late. Sure, the ball hits his hands, but when it does he's all twisted around and he's in no position to make the catch.
Brady has never gone into great detail about the play to my knowledge. WW was asked the question about where he expected the ball (
BSPN: Pats' Welker on dropped pass: I let team down ) but never answered it directly:
It seems to me to probably have been a mental error on him, since the right place to throw the ball would have been the outside shoulder, so that's where he should have expected it.
No one disputes the importance of the play, including WW himself.
We do have to give him credit for being a man and faulting himself for not making the play. Unlike others, he never thew shade at Brady, he faulted himself for not making the play.
IMHO he did screw up his legacy by going to Denver. That was a decision he made with plenty of time to think out the pros and the cons as opposed to a pass in a very difficult game, and IMHO that cost him a lot in terms of his legacy. I think in his mind he was justified in leaving, but to me that was his ego kicking in, and when that happens we don't make our best decisions.