If you look at the play from the backside, you had a TE block the the DE, and the T move to double team the backside DT, the center went to double the playside DT. Unless there was something that triggered a line call to change protection, the center would never have seen the blitzing LB, that's not his responsibility. Since Knevel didn't immediately jump to the 2nd level, I would assume Farmer would have the responsibility of picking up blitz in that gap. Mostly because he crosses his face, he could disengage and attack the LB.