Former West Germany international and Cologne star Dieter Mueller was in a coma after suffering a heart attack, German daily Bild said on Friday.
The 58-year-old, who scored a hat-trick on his debut for the national side in the semi-final of the 1976 European championships against Yugoslavia, collapsed at his home and had to be resuscitated by his partner, the newspaper said.
Thorsten Siegmund, spokesman for third-division outfit Kickers Offenbach of which Mueller was chairman, added: "We were informed by his partner. We hope that he'll win this fight."
Mueller scored 231 goals for Cologne between 1973 and 1981 and was top scorer in 1977 and 1978. He also played for among others Stuttgart and French side Bordeaux, where he won the the Ligue 1 title twice in 1984 and 1985.
He ended his career in 1989 at Offenbach and has led the club for the last 12 years.