If your looking for someone to blame, blame human nature.
In what manner? If this was natural or inherent, wouldn't every team be going through something similar?
Its impossible for a team to get up for every single game, especially a smart team like this when they know nothing matters until the playoffs start.
Blake Griffin went at full speed for the first half of the season, now look at him, he burnt himself out.
Celtics are playing smart, why go full speed only to burn yourself out and have to struggle through the playoffs?