We had a relatively healthy defense for the 1st half of the season, along with once in a lifetime talent leading it (Jaylon Smith). Smith was so good, that even with Brian Van Gorder's garbage playcalling, that the defense put up somewhat respectable numbers in the 1st half (until the UNC game).
By the time the UNC game came around, there were already quite a few injuries, thanks to Paul Longo's "strength and conditioning" program's effects, along with terrible luck.
It only got worse after the Navy game, where we lost several of our starters to those cheap near chop-blocks, especially the one that put Joe Schmidt out for the year. By the year's end, we were starting 4th string guys on the defensive line.
At that point, even Jaylon Smith's talent and abilities couldn't cover up the multitude of holes that opposing offenses definitely gashed, combined with Van Gorder's stupidity. Seriously, who in his right mind continually calls Cover 2 and Cover 3 against spread offense teams that mostly throw dink and dunk passes?