I'm not worried about the refs at all. Lebron will get his calls, that's the way the NBA works. He does not get every call, however, he gets hit every time he drives, but he doesn't go to the line every time. Yes, they don't call the travels and they do call some ticky tac fouls, but that is what happens with star players. The Celts have not lost any games against Cleveland this series due to officiating.
They lost game 6 because they missed a ton of free throws, turned the ball over too much, and let the Cavs crush them on the offensive glass. There's not much you can do about the free throws, the Celts are a good free throw shooting team, they just had a bad game, you hope it evens out and they have a good night from the line in game 7.
The turnovers and offensive rebounding, however, can be addressed. Those are issues of focus and attention to detail. Guys need to box out, they know the Cavs are looking to hit the offensive glass, they can't leave Thompson and Nance without a body on them. All five guys need to look to rebound on the defensive end.
With respect to turnovers, I have seen two issues. First, lazy passes that lead to fast breaks. This is inexcusable in the playoffs. They know the Cavs are looking to attack passing lanes, they need to avoid the careless passes. Two, trying to do too much. A lot of turnovers occur when a guy tries to make a really difficult pass. Unless you are an elite passer, you should just attempt good passes. Good ball movement forces the defense to react and move and will result in good shots, no need to thread the needle. They also need to avoid hero ball, I know Jaylen and Jayson can take a lot of the Cavs one on one, but they need to avoid trying to do it all. Even Korver can defend Brown if he charges right at him at then tries a circus shot.