The 25-year-old Brazilian has fallen down the Old Trafford pecking order in recent seasons, with Louis van Gaal opting to use Luis Antonio Valencia as his first choice right-back last season, and will hope to revive his career in France.
The Rio born defender moved to Man United in 2008 and became something of a fan favourite but his inconsistent displays frustrated Sir Alex Ferguson as well as the managers who have succeeded the veteran Scot.
Rafael was often guilty of text-book errors and there have been question marks over his discipline and the arrival of Matteo Darmian from Torino effectively ended his Man United career.
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Rafael has made a total of 169 appearances for Man United and has scored five goals in the process.