I don't know who is taking the losses lightly
Jeff Green is the first name that comes to mind. Maybe Bass. Barbosa.
Those are my guesses.
Why exactly is Barbosa a problem? He hasn't been part of the rotation for weeks. Green? Bass? Have you ever seen Jeff Green and Brandon Bass laugh and joke about anything ever? I haven't.
My first guesses would be Paul Pierce and Jason Terry.
The reason those three guys would be the first ones to mind are:
Barbosa has always struck me as a player who seems accepting of losing. A player who is happiness depends most of all on his role offensively. How many touches and shot attempts he is getting. If he is getting them, he's good. If he's not, he's not happy.
To a lesser degree, I think Bass is the same.
As for Jeff Green, I don't think he is a driven player. Lacking in competitive spirit. He's also another player who's happiness is also heavily linked to how involved he feels offensively.
I also thought Chris Wilcox was a great selection by whomever mentioned him. I overlooked him. Wilcox has played for losing teams throughout most of his career and lots of bad (mental and otherwise) habits from all the time he spent on those teams.