The Germany international moved to Chelsea on a free transfer from Bayern Munich last summer but an injury-stricken first season at Stamford Bridge saw Ballack make just 26 league appearances as United reclaimed the title from the Blues.
"I think it was a mistake to move to that club (Chelsea)," Beckenbauer told German newspaper Bild. "There was a space available for his position with Manchester United and he would have fitted into their system perfectly. The same with Real Madrid. But at Chelsea his role was already filled by Frank Lampard."
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