Liverpool chief Damien Comolli admits a player's character is crucial when assessing transfer targets.
The Frenchman admits he is constantly concerned about the impact pay rises and transfers will have on other top names at Anfield.
Comolli told the People: "You have to be careful about giving long contracts to certain players. If they have difficult personalities they could be a bad influence if things go against you.
"A guy might have big talent, but might not want to work hard. I lose sleep about this on a regular basis.
"Should we open talks about a new contract with this player or that player? I know that you can lose the chemistry of the dressing room with even one bad decision.
"When you first identify the targets, you look at the character, of course. They have to have the right approach for Liverpool.
"If everyone in the group has the right attitude, then you need leaders. They can be technical, tactical, physical, intimidating or vocal. In the case of Steven Gerrard he is all of them except vocal, while Jamie Carragher is vocal.
"Stevie is a quiet leader but the players say he carries the team."