2018 was always going to be a transition year for the Boilermaker defense. Purdue lost 9 of its top 15 tacklers from last year, including 3 out their top 4 linebackers, 4 out of their top 5 defensive linemen, and 2 of their top 4 defensive backs.
There are very few teams in the nation equipped to handle such huge defensive turnover without a substantial regression. Off the top of my head those teams are: Alabama, Clemson, Ohio State, Georgia, and LSU. In fact, if I made a list of teams capable of handling such a catastrophic exodus of talent and experience, Purdue would be somewhere near the bottom.