Perhaps immature wasn't the best word to use. Raw talent will only get you so far. Then you have to mentally prepare for what you will be doing in the games and then mentally execute the system during the games. I got the impression that Saban was frustrated with certain players at times when they were just not mentally engaged or prepared to play as well as they should have been. (especially the defensive backfield and linebacker groups) In the NFL, you don't go to academic classes and you don't have homework. Either you understand what is going down or you probably won't last very long simply because playing a certain football position should be your one and only focus. Saban is much more into strategy and execution than in player development.
