Answer
Slaven Bilić was the coach for whom Edin Terzić worked as an assistant before returning to Dortmund. Terzić served as Bilić's assistant at West Ham United from 2015 to 2017, gaining valuable experience in the English Premier League. This collaboration laid the foundation for Terzić's subsequent success as an assistant coach at Dortmund and his eventual appointment as head coach in 2020.