Zeller is the fourth worst center in the league via RPM. Only ahead of one way, black-hole, defensive sieves Seraphin, Speights and Okafor. He's second to last in BPM among all Celtic players. Dead last among Celts with at least 500 minutes.
He's far below replacement level in the two best metrics at predicting team success. It's getting to the point where Mickey might not be good, but he almost can't be worse.