Somebody is always responsible for a bootleg or reverse, and being the end man on the line of scrimmage away from the action would make that McGahee in every scenario I'm aware of. Trick plays are allowed and reverses are not particularly tricky, so I don't think any slack is deserved. His very next play (the Sorsby TD run) was also not great, he was the unblocked guy - they did bluff a block at him - and he didn't get particularly close to stopping Sorsby.
He only played like 9 snaps by my count, so it's tough to make a huge impact. But he had one very bad missed assignment, another play where he was the guy in position to make the stop and didn't, and nothing of note to my eyes on the other plays.