The most important hire to be on Chip Kelly's new coaching staff is likely to be the defensive coordinator. Chip Kelly has his offensive philosophy, and the offensive coordinator role will likely simply be implementing Kelly's offense. The quarterback coach will be important given the issues at the position. However, it's the defensive coordinator that will potentially have as much autonomy as any coach.
Matt Maiocco reported on Friday that Kelly has already spoken with Eric Mangini. There is no reported resolution, but Mangini would seem to at least be under consideration for the defensive coordinator role. It was a rough year for a defense dealing with a lot of turnover, and I would contend Mangini did as good a job as any 49ers coach this past year. There were some issues that would need to be resolved this offseason, but the defense had more positive steps forward than the offense.
It remains to be seen if Kelly will retain Mangini. It's also possible Mangini might want out, but we have not heard anything about his status, or his attempt to find a job elsewhere. My guess is he is interested in sticking around to build on 2015, but we'll likely find out more this week. Kelly's defensive coordinator in Philadelphia was Billy Davis. He was the 49ers defensive coordinator in 2005 and 2006 before bouncing between Arizona and Cleveland, and then Philadelphia the last three years.