I don't know if any one particular loss "changed" their season. I think they're just in a slump offensively. I mean, they've given up LESS than 90 points in 9 of their last 12 games, and EXACTLY 90 points in 1 of those 12 games.
You should win more games than you lose when you're giving up 90 points per game, but for some reason, they're 6-6, and it shows on the scoreboard that their offense didn't show up.
The only loss I can imagine that really "changed" their season is the Grizzlies game, because they fell further behind the Bulls for the #1 seed in the East, and Doc sat Rondo out against the Timberwolves. Does he do that if the C's lose against the Grizzlies? Did that even matter?