His first job was the second-tier Eerste Divisie club FC Eindhoven, where he worked as an assistant coach for a year.

After the club had found itself in a relegation battle under his leadership for two seasons, Koster took over the reins 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.

[7] He became head of the youth academy of Ajax, and was appointed manager of the club's main squad on 9 October 2007, when Henk ten Cate went to Chelsea.

On 25 November 2014, Koster started to work for VfB Stuttgart as field coach of Huub Stevens.

[15] He managed to qualify the team for the KNVB Cup final for the fourth time in club history, in which Ajax, however, proved too strong and won 0–4.