Lets say KP is out like 4 weeks which would be through basically the EC Finals. Assuming Indiana and New York take care of business against MIL and Phili I think Boston would still be favored against all of Cleveland, Orlando, New York and Indiana.
The first three of those teams just don't have enough offensive firepower. Boston will still have bare minimum four shooters and for about 2/3 of the game 5 shooters on the court at all times with Horford at Center. In order to beat the C's elite perimeter D you'll have to hit threes, and i just don't think those teams will do it enough.
Strangely I think Indianan might be the biggest threat. They have enough offensive poop to win a series but are also, by far, the worst defensive team of that lot.
I don't know, Boston was 21-3 without KP this year and this quality of team (good, but not elite) seems very winnable even without him.