They are clearly capable defenders. They clearly always get an edge of some kind against any team they play against, but then they do all sorts of sloppy nonsense that pretty much throws the game away for them or makes it much harder than it really needs to be. Now, why? Why is this happening so consistently?
Celtics, with the way this game has gone, should easily have at least a 5 point lead or better for a good stretch of this game. They are a very good team that makes it way too easy for other teams. They almost enjoy playing from behind, because they are afraid of the expectations that come with leading (They are expected to win, and they can't handle that expectation, not anymore).
And how else do I explain their sloppiness? Their consistent throwing up on themselves? This team, every last player on the team, does not actually believe they will be able to compete in the playoffs, much less win a championship. This team is playing like a team that knows they aren't good. It's their pride that forces them to fight back into games before giving up again.
Going further, you can't just say you want to win. You have to show that you want to win. These Celtics do not want to win. If they did, they wouldn't make the kinds of mistakes that they do, the silly turnovers when they have other teams in trouble. They seem to play like a team that can't stand the sight of mid-game success, and really isn't interested in working hard enough to maintain it. They squander opportunities in a fashion similar to the way the worst teams in this league do.
So, forget what the celtics player say, forget what the coach says, this team has given up. They don't want to work hard enough to be winners. They use to back up their talk of hard work with real effort. This team isn't trying. The collective whole on this team only suit up every night for one purpose and one purpose only, to not go out there and get embarrassed.
Why else would they be so amazingly consistent at dropping leads when they get behind? Why not put in that kind of effort when the game is close, or when they have a small lead? They don't care to. They are out there to avoid blowouts, not win games or win championships. This may not be true of specific individual players, but as a team it is.