BERLIN — Top execs at the Bertelsmann Music Group arrived at work today and faced a major challenge: replacing Rudi Gassner, one of the music world’s most powerful forces, who died Saturday in Germany of a heart attack.

The 58-year-old Gassner was set to take the reins at BMG on Jan. 1, replacing CEO Strauss Zelnick, who ankled his post in November following a management dispute with Thomas Middelhoff, chief exec of parent company Bertelsmann.

“The death of Rudi Gassner is a tragic loss,” Middelhoff said Sunday in a statement. Gassner “was a successful media entrepreneur and a great hope for us all in view of the necessary development and reorganization of our music business.”

Bertelsmann announced last month that Gassner would be returning to head BMG as president, chief executive and chairman-designate starting Jan. 1. Company says it will decide on Gassner’s replacement early next year.

His death comes as BMG and Bertelsmann are in merger talks with U.K. music giant EMI.

Gassner’s career in the music industry spanned 31 years. He most recently served as president and CEO of BMG Intl., where he was credited with transforming a fledgling group of music companies into a cohesive and successful international music division.

Last January, Gassner left BMG after 13 years following a series of disagreements with Zelnick. He then moved to indie record company Edel, where he served as chairman of its supervisory board. Before coming to BMG, Gassner spent 18 years at Polygram Records. In addition, he was an active member of the Intl. Federation of the Phonographic Industry (IFPI) as well as an adviser to the dean at Harvard’s John F. Kennedy School of Government.

A native of Munich, Gassner was a professional soccer player before he went into the music business. He had lived in the U.S. since 1987.

Gassner is survived by his wife, Brook, and four children as well as his mother.