I believe it has been reported that they (coaching staff read BB) considered him a coach on the field when he played, so moving to an actual position coach should be a no brainer. This was BB's quote after Mayo retired.
"I'll start out this morning, first of all, to talk about Jerod Mayo," Belichick said. "I talked to Jerod a few weeks ago. He told me about his retirement. There have been very few players in my career that I've had the opportunity to coach that I'd say had more of an impact on a team than Jerod has from day one, which is unusual. From the first day he walked in the facility, which was his pre-draft visit, after we drafted him in 2008, he's been a pleasure to coach, a great addition to our team, both on and off the field," Belichick continued. "I'm sure I learned a lot more from him than he did from me. Jerod, (Chantel) and their family brought a special glow to our team, to our organization. Although Jerod will always be part of the team and is always welcome, he'll be missed on a daily basis, the attitude, work ethic and love of football that he brought was special. He was very special."