Couple of things to echo here from other posters. It is true that sometimes great players have a hard time becoming great coaches or managers because what made them so great was a mix of God given talent, or a fanatical devotion to details which a lot of the players they interact with will not have and will have a hard time relating to. WHY can't they do what I do? WHY don't they study film like I did? WHY aren't they living in the gym?
I don't know as much about Mike Singletary as I do about Ray Lewis simply because of the era and the fact that he played for Chicago. I know he was a beast on the field, but I don't know how much of a film buff he was or what kind of teammate he was. I think it showed in a couple of press conferences that he couldn't relate to some players. I am speaking of the one where he dropped his pants and mooned the press with boxer shorts on and the other one when speaking about what he thought of Vernon Davis, "Player like that....can't win with them.....can't do it."
Ray was a team captain and essentially coached the defense on the field. He held his own personal film study sessions outside of what the coaches did and Ed Reed was also in the room with him all the time. As a player when his physical side started to diminish he willed this team to victories and it was his superior game knowledge that allowed him to play at a high level.
As a motivational speaker I think he could fire up any defense or team he coached for. It is a dual edged sword though. Some players with the Ravens grew tired of his often sermon-like speeches and I'd have to think if he was coaching a losing team the players would get tired of hearing his at times rambling and nonsensical utterances.
His time as an analyst never really took off which probably means that career is short-lived for him. If he wants to be close to football again after his children are all grown up, coaching will be his in. I do think it would be a mistake to hand him a franchise right off the bat, but I can certainly see him as a "guest" linebacker coach at first just like the Ravens did with Singletary. See how he does, see what impact he has, then move him up the chain from LB coach to DC, then possibly a head coach if that's what he wants to do. Knowing what I do about Ray, I would never bet against him to do anything he wants.