The 23-year-old will spend the coming campaign at Gelsenkirchen after impressing on a loan spell last season.
The Ghana international wants to spend time with the Royal Blues as he seeks recover from a serious injury which has kept him out of action since January.
Chelsea are believed to have reached a loan agreement with the German outfit which will keep the left-back at the club until June 2018.
The move is in no way a precursor for an exit for Rahman, but is designed to give him more experience of first-team football.
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