Fault implies there is something that requires blame. We lost two basketball games. That's normal. Miami lost, San Antonio lost, Oklahoma lost why are we any different? There is no "fault" we lost to teams that played better than us.
What you actually mean is why does the teams play not match the expectations you had of them before they actually played? That is easy, your expectations were unrealistic. This Celtics team is a great team. They will be a great team when it matters. But for anyone to expect this team to be a great team from game 1 to game 82 is unrealistic.
Sullinger will improve by leaps and bounds (not literally) as he gets used to the NBA pace and personnel. Jeff Green will get better the further he gets from missing a season due to heart surgery to actually playing and competing for a championship. Terry will become better once he understands his role in the Boston offense. The team will get better as Terry, Lee and others get used to Rondo (and KG). Doc will get better as he learns more about what works and what his players are capable of against NBA teams.
At the end of the day, we're going to win a lot of games and compete for a championship. Rondo is better than last year. Bass is better than last year. KG and Paul continue to look great. Avery Bradley is another year older, extremely confident, hungry and hopefully healthy. The talent in our bench is incredible. If Kobe, Nash, Peace, Gasol and Howard struggle learning a new system playing 40 min a game, why wouldn't our bench which has 5 new players playing only 20 min?