The 30-year-old German was not punished by referee Lee Probert following the incident during the Cottagers' 3-1 defeat to United at Craven Cottage on Saturday.
Januzaj was attempting to run down time in the corner late in the game when Riether and team-mate Pajtim Kasami tangled with the youngster as they attempted to win back the ball.
The incident was missed by both Fulham boss Martin Jol and United manager David Moyes but the FA could take retrospective action after reviewing video footage.
"That would be disappointing," Jol said about a potential ban. "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."