The incident occurred towards the end of Saturday's 3-1 defeat at home to United, but was not seen by referee Lee Probert or his assistants.
However, Riether has not escaped punishment, becoming the first player to be charged by the Football Association under a new pilot scheme where a panel of three former referees reviews video evidence of incidents not seen by match officials.
The German defender had until 6pm on Tuesday to respond to charge and took to Twitter ahead of the deadline to accept punishment, as well as apologise for his actions.
"I want to say I am sorry to Adnan Januzai," he said via his account, @saschariether. "It was very unlike me and I can only say that frustration got the better of me.
"I also want to apologise to my teammates, fans and everyone at the Club, for the ban incurred following Saturday's match.
"I will accept the consequences of my actions, and hope to put it behind me when I return.
"I want the fans to know that I will be working hard to stay fit and will be ready to continue fighting for the Club as soon as I am able."
There were reports that Riether could escape punishment for the incident thanks to a precedent set when Chelsea's Fernando Torres avoided punishment for scratching Tottenham defender Jan Vertonghen's face.
It is understood Fulham had looked into such a loophole, but chose not to appeal the sanction.
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- Lee Probert