The route concept is a double move deep post with a go route option if the defense isn’t in Cover 2 (two safeties up top before the play started).

Welker runs the double move perfectly, going right by the DB, so now it’s time to run his post if it’s Cover 2. Except it’s not Cover 2, it’s Cover 3. Based on the route concept he should option into the go route and run straight.

Wes however doesn’t read the top safety being in Cover 3 and not Cover 2, and he starts his post route and looks for the ball. Of course Tom already started throwing the ball for the vertical route since it’s Cover 3, and Welker’s now out of position. He tries to contort his body for the grab but he’s not in position.

If you can find the All-22 footage, and draw a straight vertical line from the double move, you can see Brady’s throw was near pinpoint accurate. Welker’s lack of the read is what ruined the play, not Brady’s throw.