The 32-year-old Schaefer, who spent 10 years at Wolfsburg and helped them win the 2009 Bundesliga title, played 256 Bundesliga matches, scoring 13 goals.
He also won the 2015 German Cup and the German Super Cup that year.
Schaefer will return to Wolfsburg by 2019 at the latest to join the club's management team.
(Reporting by Karolos Grohmann, editing by Ed Osmond)
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