To Harpers defense Luke seemed to be in the gap up the middle and then he bit on the fake by Bell. Once he bit he got swallowed behind the block. When Harper saw Luke on the inside gap he committed to filling the gap on the left then Luke bit on the fake as I mentioned opening up the hole. I know we all love our middle linebacker but that play looked like Luke's fault to me.
From what I think I read a few days ago, that was Luke's assigned gap and Harper was a free-roamer whose job it was to diagnose the play and pick which gap needed filling.
Regardless they have to work those problems out.