You need an offense that can consistently generate a lot of quality three point looks and a steady parade of free throws. That's what wins you a lot of regular season games.
In the playoffs it's about having the firepower to hang with any opponent until the end of the game. Then, in crunch time, you need to be able to clamp down for a few key defensive possessions while grinding out points on the other end with individual scorers that can't be defended 1-on-1.
Right now the Celts don't have the offensive firepower or the consistent shot distribution / generation to be able to reliably win regular season games.
As for the playoffs, they probably still have the capability of playing strong defense for a few possessions at a time. Problem is, again, the lack of firepower to be able to consistently keep it close until the last couple of minutes against good teams. Even if they do keep it close, they've shown that they are really terrible at executing late game possessions.