I really don't like Tyler Zeller.
Yes, I love his hard work, and quick motor. But hes a back up center. So why are we thinking of extending a back up center, especially if we don't have a starting one, and our chances of getting one are slim to none? Way I see it, Zeller will either try to resign for a very team friendly option contract, or he'll just have a good contract that can allow Ainge to flip him to another team.
I'm not set on Zeller, and really don't want to re-sign. If we had acquired a dominant center, and Zeller was still here, then sure, extend him all you want, but hes not a starting caliber center.