Three years after the end of his playing career, Koster began his managerial career. His first job was the second-tier Eerste Divisie club FC Eindhoven, where he worked as an assistant coach for a year. He then went to the Eredivisie club Willem II, where he worked alongside Piet de Visser. After the club had found itself in a relegation battle under his leadership for two seasons, Koster took over the reigns as head coach. This did not bring a change, which meant that he was dismissed after barely a year in charge, and he was again replaced by De Visser in 1991.