The German accepted a charge of violent conduct after his clash with the Red Devils youngster, with his suspension to begin with immediate effect
The German has accepted the suspension, which begins with immediate effect, and is set to miss his side's upcoming games against Liverpool, Swansea City and West Ham.
The Football Association (FA) confirmed the news, with Riether the first player to be charged and punished under a new retrospective action scheme, after the incident was missed by match officials.
"Following an Independent Regulatory Commission hearing [on Wednesday], Fulham's Sascha Riether has been suspended for three matches with immediate effect after he accepted an FA charge for violent conduct," a statement on the FA's website read.
"Riether was charged by the FA following an incident which occurred in added time of Fulham's game against Manchester United on Saturday, November 2, 2013.
"The incident, which involved United's Adnan Januzaj, was not seen by the match officials but caught on video."
Riether had earlier apologised to both Januzaj and his team for the incident, claiming that frustration had got the better of him.