Honestly guys, that's not a mistake by Foles. You're looking at it from hindsight. Foles has to use foresight. Look at the pre-snap alignment. You have 4 defenders aligned to the right-side of the offense. You have one defender in the middle. And you have two defenders to the left. Welker's defender is giving a 10 yard cushion on 3rd and 7. It's a quick timing pattern. Foles isn't supposed to go through his progressions. He made a pre-snap read and went with it. It wasn't the wrong read based on what was presented pre-snap.
And the play would have worked if not for two mistakes. One...Welker was rusty and didn't appear to get proper depth on his route because when he rolled out of his break, he came up a yard short of the marker. I think he was supposed to run that to the sticks on that play. Two...Foles's pass was just a little off which kept Welker from being able to turn up field after catching it. If either of those two things happen, it's a conversion. If Foles is just a bit more accurate, Welker might be able to turn it up for a TD.
So while yes, he technically missed a possible TD. It's not a miss that can be held against him because he read it right, he executed the play, and things just happened to work out in a way that couldn't be anticipated. Now, if this were a longer developing play where he had time to go through his reads, I'd agree with you. But with this being a quick timing pattern, Foles can't take the time to go through his progressions and scan the field or else the ball will be coming out late...which leads to a Pick Six on an out route.(and frankly, him getting the ball out late has been a consistent issue this year)
EDIT: Looking at the play again, I have to change my opinion. It was the throw. Welker was trying to roll up-field, after coming out of the break, to the sticks but Foles's throw led him back towards the LOS rather than up-field.