Yes.
If Banker can hold Rutgers and Iowa scoreless and to -875 yards total offense, then yes, there is a slight chance his defense could finish in the top half. If all other teams score in the triple digits against each other for their remaining games, a top ten finish could be the result.
The question wasn't "Will Nebraska end up in the top 5-15 in major defensive categories for the 2015 season." It was is he a top 5-15 defensive coordinator.
IMO, still probably not.