Depends. If the roster stays as is, then no.
While I think the team plays better without Rondo, I don't think this roster can beat Miami or Chicago.
Now, if Ainge can land Josh Smith for Bass, Green, etc, that changes the whole situation. Suddenly, KG has another very competent PF to back him up in the front court. And if the Celts can also land another wing who can shoot, even better (imagine if they can land Kyle Korver with Smoove.)
BUT, if the Celts can't land a player who can help them win now and a good offer comes along for either KG or Pierce, I would say take it.
The goal should be to win a championship, not just make the playoffs and get bounced in the 1st or 2nd round. Therefore, you should build toward putting together the next championship caliber team.
Another thing to consider, if the Celts realize the "rebuild" route is the way to go, they should shed as much salary as possible (Terry, Lee, Bass, etc) and go into the summer with tons of cap space (to go after Howard and Smith.)