So I covered a bunch of good things from Borland in this game. That was my version of a disclaimer before I cut this play up, because I thought he had a hell of a game. Here are Borland's comments regarding this play:
"They ran vertical routes to my right and a drag to my left, and I thought I had help, but I should have known I didn't (have help) to the outside," said Borland, who explains the Xs and Os of his assignments unlike many players and coaches in the NFL. "So the drag, I should have covered it." source article
2Q 2nd & 12
STL: Shallow Cross concept.
49ers: 5 man pressure (Wilhoite blitz) with some sort of pattern matching from cover1. It's a cover2 look only at presnap. Bethea and Cox may be playing Quarters.
Color coded are the two routes that end up causing the confusion for Borland. Britt's yellow drag and the RB's gray check release. Notice the two vertical routes being defended by Cully and Ward. The term "vertical" is a loose term used in pattern matching. It can mean anything over 7-10 yards, depending on how the coverage is taught to the players.
This is the moment in time where Borland could have recognized the Drag route was his and matched it. With Cully and Ward being taken vertical, there is no one else to cover anything on that side of the field. Not that Borland could have known Cully and Ward were being taken vertical, but he should have known that Bethea was behind Borland to pick up any remaining route, in this case, the RB. Understanding where help is located is vital to properly pattern matching. How many WRs can you let go by you until you must follow the next WR? In this case, for Borland, it is zero.
Bethea has no interest in following the WR that is running by him. That is Cox's assignment. Borland hesitates because from his point of view there are two routes coming at him. The Drag (yellow) and the RB leaking out (gray).
The drag becomes wide open.
You can see Borland take one step to his left, towards the RB's check release. He needed to match the more immediate danger - the drag. I'm not sure if Cox/Bethea should have given the 'Under' call to help Borland. If that actually took place, or if it should have taken place.