With the amount of adrenaline they're going to get from a Game Seven, from the Boston crowd, and from national interest ... I don't energy is going to be an issue next game.
However, I do agree with you, the Celtics looked very tired in Game Six and not for the first time in these playoffs. If they get past the Cavs, I'm worried about it.
Kevin Garnett stood out the most for me last night. I said in the game thread, he was barely getting off the ground on his attempts around the rim. On two plays it looked like he went up to dunk but didn't elevate enough, had to change plans in midair and ended up blocked and missed the other. He's looked like a couple of times this season, but never at home, so I don't expect a repeat tomorrow.