KG's major injury, nothing more nothing less.
He was out one playoff run and relatively hobbled as far as rebounding and other aspects of his game for another playoff run (one in which the C's made game 7 of the Finals ANYWAYS)
KG got 4, 4, 6, 6, 10 , 6, 3 rebounds in the 7 games while playing 30+ minutes in all but one of them. By the next year he'd recovered fully and was more in a natural level of decline due to age, but Ray Allen/PP's decline set in more severely...
Even considering that, the team acquitted itself very well. The Celtics franchise has won so many close games in their many other titles, we came out the wrong side of one during the first three years when we were setup to be the best team in the league. Sucks but it happens.