Green is probably as valuable as Pierce is right now when you consider age and contract.
Sulli is risky long term because his health concerns.
Melo has little to no value on a team trying to compete now. And even less if you acquire a possible all star center in the process.
This trade would actually be amazing for the Celtics. And might be just what they need to get past the Heat who's only bad position is center.
Yes, Garnett might cry for a while... but winning solves everything. And you've already committed to Green.