Its not a fiery leader that they need, its an enforcer.
Someone on your team, that the other team knows will knock the other teams 'hot' player down, if they mess with your superstar, or even when they do not. Metta World Peace was very vocal about this when he joined the Lakers. He said he joined the Lakers because Kobe needed protection. You cannot have your star player also be your enforcer. You need a 'bad guy' on your team. A Metta, a Barnes type.
Boston lost this when Posey left, and when Perkins got injured. Just the scowl on Perkins face made opponents fear and respect the Celtics. After these two left, guess who became the enforcer on the Celtics? Rondo !!!! He was also given the keys as the leader. That is counter-productive. You cannot have your leader be your enforcer. I remember when Rondo tangled with Humphries, and I thought 'really, you are going to have your little guard go after a 6'10 guy? That should not have been Rondo's role.
The problem now is the coach does not understand the need for this type of player on his team. He wants 'good character' types that do not strike any fear in him, and therefore do not strike any fear in the opponent. If all you get on the team are players that mild mannered CBS does not have to worry about, how are these players going to strike any fear or respect in tough bad ass NBA players. Look at GSW do you really want to mess with Durant, Curry, or Thompson, and worry about Green or Ighodala giving you a hard foul? There is a reason Barnes always finds a landing spot - toughness.
Marcus Smart can do it if he is unleashed by the coaching staff.