Zeller is capable on offense, he runs the floor well, solid in pick and roll, he can pop out to 15-17 ft, he understands where and how to set picks, and keep the ball rotation moving.
It's defense... he's soft, he's not a good rebounder for being 7 feet, opposing bigs that are not known for their offense have career nights against him. He doesn't box out well.
He's an ok backup center, at best.