the best managers seem to have had average ability as a player and therefore worked the hardest to stay in the team. Jack over Bobby Charlton for example, Bobby one of the best players ever yet poor manager. Jack a bit like Carra really.
Thats why I see Carra being a success over a Gerrard for example. Carra has done evrything to extend his top flight playing career. Nothing cames easy to him.
I also think this is where Robbie lost his way, all was very easy for him when he was younger such was his ability. he should be top flight still now if he had pushed himself harder in his early twenties.