Just to add my dos centavos, several things come into play here with Carlos Lee:
1. Is he known around the league as a lazy player? Not that I know of, so if he's all of a sudden decided to be lazy, it's probably a very new phenomena.
2. Is he human? Yes, he is. He's not a robot, so he's prone to make errors both in execution and judgement.
So having said that, there is making an error in judgement that is benign in nature and there is making an error in judgement simply because you just don't care. If it's the latter, then managers have a way of sensing that and will do something about it. Guys like Bobby Cox have pulled players right off the field for dogging it and it was precisely because they felt the player had an attitude that needed adjustment. Same with Lou Pinella this year when he pulled Soriano right off the field for his lazy plays in left one inning in Chi-town. Until the results are in that Brad Mills feels Carlos Lee is absolutely in need of an attitude adjustment, then I'll join the crowd that Lee is a huge problem for the team.
Now, is Lee taking some of his early season frustration out into the field with him? Absolutely and because of that a manager will try to get a good player like Lee to re-focus. Sometimes it's harder to get a player to let things go, so you sit them for a while or juggle the lineup so thinking is taken out of the equation for a while. Lee's problem hitting, to me, is a product of not having anyone behind him nor in front of him he could trust to do their job. When you have free swinging Pence in front of you and free swinging Feliz behind you, you're prone to start getting out of your game and trying to do too much. This may be a case of actually caring too much to try and help the team win early.
In terms of losing focus on the field, on the basepaths and on the help Lee should provide a runner... chalk it up to his own frustration spilling over to the other facets of the game. Players will forgive a lack of focus sometimes when they know you're struggling. But they'll also expect you to pull yourself out of it and not make it a habit either. Good thing Mills is on the job though, he knows when it is time to do something about it and when it's not.