Couldn't be too soon...
IMHO the return of Bradley is the pivotal point for this team. If I were the GM I would remain patient until Bradley returns, then give it about 10-15 games to see how contributes and how the team response to their new roles. It's possible we could be much stronger once we have Terry, Lee AND Green all coming off the bench - combined with Sullinger and Wilcox that's a very strong second unit, and maybe that alone could be enough to transform us.
If Bradley returns and returns to his previous season's affectiness after the first 10-15 games but the team STILL continues to struggle, then I'd put Collins, Lee, Bass and Barbosa out on the trading block and see who bites for any combination of those guys plus any number of picks. If it's needed to help sweeten a deal and bring a consistently productive player here, I won't think twice.
I don't care if it's Pau Gasol, Josh Smith, Al Jefferson, Paul Millsap or Marcin Gortat - as long as we get somebody who is going to come out and give us consistent 10/10 production (give or take) every night I am happy.
Either one of those guys is going to bring close to a double-double average, and having another big that you can DEPEND on to give you scoring and rebounding every night will take tremendous pressure off KG and Pierce to have to force things and carry the load. Plus our depth would not suffer much because we'd still have Terry, Green, Sullinger, Wilcox and whoever else we get in the trade (plus Joseph and Melo available for Call-up).