After winning promotion to the Bundesliga in 1979, Hörster was an essential defensive unit for Bayer in the following 13 seasons, his lowest input being 24 games in the 1981–82 campaign. In 1987–88 he appeared in a total of 35 official matches, including nine in the team's victorious run in the UEFA Cup – one goal against FK Austria Wien in a 5–1 home win (also the aggregate score)– even though he did not appear in any of the two matches of the final against RCD Español.