Part of the reason for Boston seemingly struggling is that no one cares about the regular season anymore, not even the players. How teams do in the regular season has never been less informative about the teams' actual strength.
There was an outrage when Lebron's Miami lost a few games, but today the Brooklyn superteam could end up in the 6th seed and no one would blink an eye. This means that there is less pressure on the players to perform during the regular season, and it doesn't help having no fans and feeling like the previous season just ended.
Boston is a good team, but not quite as good as the very best ones. They will go about as far in the playoffs as they have in the past few years, which will be a measure of their true ability.