Well we all know that the celtics worst positions are at the small forward and at center. I think two reasonable options are Harrison Barnes and John Henson. Both are still young and talented, and probably wont needed to be traded at a very expensive price.
The case for Barnes:
Out of high school Barnes was the number 2 player in his class but was given some extremely high expectations to live up to. So far Barnes has not been the dynamic scorer that people thought that he would be, but it has been hard for him to get many touches with klay thompson, curry, green, and igoudala. With all of that, Barnes has still proven to be a decent shooter, a great finisher, and an excellent defender. He also won't need to be traded at a huge price.
The Trade
Celtics receive: Harrison Barnes, James-Michael-Mcadoo
Warriors receive: Kelly Olynyk, Nets 2016 first round pick,
The warriors just traded away their best backup big man and now need another one. Kelly Olynyk is probably going to be traded someday with the Celtics crowded frontcourt. Also Evan Turner probably won't last as a starting small forward anyways. Mcadoo is a very raw and athletic scorer and is a decent cheap alternative from Olynyk. For the Warriors, they would be willing to trade away Barnes because first, Igoudala is really starting to get into his groove, and it looks like he'll be their starter next season.
The case for John Henson:
Earlier this year, a trade for Henson seemed very hard to do because he still had a chance to start. But now with Greg Monroe, Zaza, and Plumlee, it seems like the Bucks can afford to give up Henson for some other assets. Henson is a very underrated rim protector and is a good scorer and rebounder. While only playing 18 minutes a game he's managed to average 8 pts, 5 boards and 2 blocks a game while having a 18.08 PER.
Three Team Trade
Celtics receive: John Henson, KJ Mcdaniels (S&T)
Bucks receive: Evan Turner, Jonas Jerekbo, 2017 76ers 2nd round pick
Rockets receive: Tyler Ennis, 2017 Heat 2nd round pick
Trade (option 2)
Celtics receive: John Henson, Rashad Vaughn
Bucks receive, Evan Turner, Jonas Jerekbo, 2016 Nets 1st round pick, 2016 Heat 2nd round pick
In the first trade, the celtics could do a sign and trade for KJ who is an athletic small forward with a lot of potential. Unfortunately for him and the Rockets, he rarely got any playing time and that may lead to him getting traded. With MCW, Bayless, and Vasquez, Tyler Ennis is destined to be traded and the Rockets have always needed a point guard for a while. Evan Turner could add some guard and forward depth for the Jabari Parker and GA. Jerekbo is a stretch four who is a good defender and has a decent contract. In option two, Vaughn is a decent scorer, but was picked 17th and the Nets 1st round pick will probably make it into the lottery so it's probably of more value.
The Lineup
Smart/Thomas/Rozier
Bradley/Young/Hunter/Vaugh
Barnes/Crowder/KJ Mcadniels
Sullinger/Lee/Mickey
Henson/Johnson/Zeller