It is on Manhertz, it's also on Cam. Cam couldn't handle the jailbreak blitzes they were throwing at the team. They simply sent more rushers than there were blockers. When that happens there is always one or more uncovered receivers, the QB has to find them, fast, and fire. If you win it, you probably score big. If you lose, it's a sack. Most QBs coming off of their break aren't up to game speed yet and Cam didn't pass the test.
The point is, those kind of blitzes are rarely done in the preseason, kind of a gentleman's agreement between coaches to keep from getting QBs injured. Guess what Belichick does? He goes after your weaknesses no matter when it is. He doesn't play by unwritten rules (or written ones when it suits him). And now we have an injured QB.
If you really look at the original play in question, the jailbreak blitz is obvious with seven men rushing the line and an eighth joining in quickly after the snap. Before the snap, Cam sees the situation, calls an audible and begins to drop back. His drop back is deep and he realizes the pressure has broken with Manhertz getting beat to the outside. Others were getting beat and gaps were being shot. He starts to scan but doesn't get enough time to find his hot read. Sack.
I'm chalking it up partially to Manhertz who I don't think is really that great of a blocking TE, and to Cam who in later season form would have dropped back two steps and shot the tight gap to Manhertz's left and made a few yards. He may have also been able to catch his hot read with a fast pass.
It's preseason, though, and the team isn't playing up to game speed yet. The Patsies came in gunning for him. They sacked him there and eventually hurt Cam in the preseason yet again. I'm sure Belichick puts that as a positive in his ledger for how the season plays out.