Saints to interview Aaron Glenn on Jan. 10th [Nick Underhill: Glenn probably gets an in person 2nd interview in a couple of weeks]

RJS said
They have had at least 12 defensive players on IR and they're still 7th in points allowed. That's overachieving IMO.
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There is also this from DerrickB's article:

Points allowed per drive and DVOA posted by Rouxble are probably a better measure that PPG allowed or rushing or passing yards per game. Glenn has done a great job this year considering their injuries they have had this year.

https://sportsview360.com/nfl-top-10-head-coaching-candidates-power-rankings-and-analysis/

I'm not sure that Glenn is some amazingly innovative defensive mind, and I'm not even sure he's the best HC candidate, but he has improved Detrot's defense greatly so he is very good on his side of the ball. But, I think it's his leadership skills and ability to build a team culture, that are the best argument for him to get the job.