Belgian ace Fellaini missed Saturday's 2-1 win over Sunderland after suffering the wrist problem in training.
Manchester United boss David Moyes revealed Fellaini will have the injury looked at on Monday to decide whether or not he links up with Belgium for their upcoming World Cup qualifiers.
"He's got a wrist injury. He's going to see a specialist," Moyes told The Mirror.
"It doesn't sound too good but we'll wait and find out on Monday."
If United's worst fears are confirmed and Fellaini requires surgery he could face at least three weeks out of action.