When the ABL folded the next year, the St. Louis Hawks acquired his rights but then surprisingly cut him. Siegfried considered retirement, becoming a high school coach and teacher before former college teammate Havlicek convinced coach Red Auerbach to try him out for the Boston Celtics. Slowly regaining his confidence, Siegfried proved to be a key pickup. He eventually became a starter next to Havlicek or Sam Jones in the backcourt. His defense and free throw shooting were key to NBA title wins for Boston in 1968 and 1969. Boston announcer Johnny Most often noted his tenacious defense, calling 'Ziggy's in his shirt tonight' to describe Siegfried on many nights.