Whilst some people are very happy bringing that kind of personality into the dressing room, i'm sure there are a few a bit worried about his lack of coaching experience.
I'm not too worried because he will have no say on the playcalling, i think he will also have very limited say on personnel selections that responsibility will probably be Capers's. What he is expected to do is to get the OLB's ready to play the 3-4 and use his vast experience in his position to help these guys. He will be such a big presence in the dressing room that he will fire these guys up.
Also since he is a former player and should have the respect of our players, maybe McCarthy thought he would be ideal as you need a coach that a player is comfortable talking to when there's a problem. Greene could be the middle man between the players and the management. This could be very good for player/management dynamics and team spirit.
The hiring of Greene is not without it risks, i mean what if he is unable to coach at all!!? but he has clearly shown passion and has 15 years worth of knowledge and experience at OLB. It could be an inspired hire if it comes off.