The full-back was hit with a violent conduct charge on Monday following an incident that occurred in injury time of Fulham's 3-1 defeat to United at Craven Cottage on Saturday, sparking unsavoury scenes between the two sets of players.
Referee Lee Probert did not punish Riether for the challenge at the time, but he has since become the first player to be charged under a new retrospective action scheme.
After reviewing video evidence of the incident, the FA elected to charge the former Hamburg man, who is now facing a three-match ban after confirming that he will accept any punishment that is handed down
The German wrote on his official Twitter page: "I want to say I am sorry to Adnan Januzai. 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."
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