This is just a long stretch what I think they should do.
First in Free Agency:
The two major areas the Celtics need, a rim protector Center and a true Small Forward.
DeAndre Jordan should be number one free agency we need to go after. Does everything we could ask for him being our center for years to come.
Kawhi Leonard is our answers at small forward. He's a final MVP, plays defense and can be a top scorer with the ball in his hand. These two players give us everything we're missing.
The first thing is a long stretch with trading up to the number 1 or 2 spot of this years draft. I know right now people are saying you're crazy. Listen, we need an impact rookie now, not later. We've gotten a stock pile of draft picks that need to be used for something. The talent I want is Jahlil Okafor, a true offensive power forward and will only get better. So this is why we need to move to the number 1 or 2 spot depending. The Knicks at the number 1 has a lot of needs and we have draft picks and players to do so. We should offer up The two first round draft picks in this year and one to two of the 1st rounders we have in the following years. This won't be it, so we'll also send Sully, Zeller, and Pressey. This would give them instant helps plus 2 first rounders and more numbers one's in the future.
Keeping two of our own by resigning, Bass and Crowder.
This is why if you break down how the roster would be after this.
Center - DeAndre Jordan, Kelly Olynyk
Power Forward - Jahlil Okafor, Brandon Bass, Jae Crowder (Can also play the SF)
Small Forward - Kawhi Leonard, Evan Turner
Shooting Guard - Avery Bradley, James Young
Point Guard - Marcus Smart, Isaiah Thomas
Other free agents you can use to finish off the roster. I know this is a long shot, but I think this would be the best option.