I want to put Andrew Bogut here but I doubt the Dubs will give him away unless they get a replacement Center who can defend as close as he does.
With that said, I will drive the badwagon and make a stop here.
Jason Smith.
Albeit his season got cut short by injuries, in 27 minutes, he averaged a solid 6 rebounds, 1 block and 10 points.
He's not the sexiest of names, but like Robin Lopez (impact wise), he's solid defensively, has size and fairly athletic to catch up the quicker Centers. Challenges shots and not scared of contact. He could come in for maybe the MLE too. Relatively young (27). And, you can slot him as a tall, big PF (for what it's worth).
And as for fit, he's a guy who shoots a great 47% from 16-23 feet (per basketball-reference.com), and that's out of 173 shots in 31 games, there's a big sample size there. He has a deadly mid range game (as posted by a poster before, forgot your name, but all credit on that quote goes to you), making him deadly with Rondo as a pick and pop option, or just opening up more space for Sullinger to work down low.
Again, he may not average a double double (or maybe he will) but he's solid pickup option for us. He'll be an excellent 3rd big off the bench, or a decent (at worst) to a good (at best) Center.