With the Red Devils winger running down the clock in the corner, Riether appeared to kick out at him as he and his teammate Pajtim Kasami tussled with the 18-year-old.
Referee Lee Probert did not punish the German, and therefore the FA are within their rights to analyse video footage and take retrospective action, if they feel that it is necessary.
Cottagers boss Martin Jol claimed that he missed the incident and told reporters regarding a potential ban for Riether: "That would be disappointing. If it is true, then we have to talk to him.
"I have got some good back-up players as well, so that is not a disaster. The result is a disaster."
The home defeat to United on Saturday left Fulham 15th in the Premier League.