The club did not give any further details but German media said he was in a coma.
The 58-year-old played 326 matches during eight seasons with Cologne, scoring 231 goals. He was the Bundesliga's leading scorer in 1976/77 with 34 goals and the following season with 24 when they won the title.
In 1977, he scored a Bundesliga record six goals in the 7-2 win over Werder Bremen, a feat which is not recorded on tape as television staff had gone on strike that day.
Muller made only 12 appearances for West Germany but scored nine goals, including a hat-trick in a memorable European championship semi-final against Yugoslavia when they came from 2-0 behind to win 4-2 after extra time.
He also played at the 1978 World Cup in Argentina.
Muller started and ended his playing career with Kickers Offenbach and also had stints at Saarbruecken, French side Girondins Bordeaux and Switzerland's Grasshoppers.
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