I want to be clear, it was a terrible loss and even worse given the circumstances. Lot of guys sucked.
But if you're going from "they are winning the Finals" to "they aren't anymore" because of one single regular season game, then I'm sorry I just can't take you seriously. It's the middle of the season and close to the All-Star break. Even the great teams in history have a game or two like this during the season and sometimes sleepwalk a few games. The C's have still mostly taken care of business and have the best record in the NBA and a 5-game lead for the 1 seed in the East.
Obviously if they have another bad week with a few blowout losses mixed in, I'll become concerned. Not now though. Yes, some of the things we've seen the last week are similar to what we've seen in recent years, BUT this is a new team and the playoffs are still a few months away. The playoffs are where this team will truly be judged but even if we had won yesterday, we'd say the same thing.
If you suddenly think they aren't a Finals-caliber team just because of one single week, then you never really believed in them to begin with. They are on pace for 60+ wins. 60 wins still means 22 losses during the regular season, and guys sit out some games too. They won't be perfect every game. The 2008 team finished 66-16, which again, that's 16 losses they had and I remember a few of them being ugly too.