What we have in this picture is Webster in coverage on Fitsgerald at the top of the screen with safety help over the top (off screen). The line backers are shifted to the bottom of the screen in a "under" formation. It's the job of the weakside DE to cover (or "contain") any runs to the outside. If the running back is allowed to turn the corner he's guaranteed a large gain as the deep safety is going to be the only defender in the neighborhood to stop him.
Instead of containing the run, Osi tries to cut to the inside (red line) which is exactly what the LT wanted him to do. He should have taken the blue line route which would have forced wells to make a decision to try and outrun Osi by moving laterally instead of down the field (allowing the rest of the defense time to adjust) or cut back toward the center of the field toward the majority of our defense.
The result is Wells getting to the corner instantaneously with only one player to beat.
Osi isn't a bad player. He's a great pass rusher but almost equally as bad against the run.