Mitrovic missed last weekend’s 2-1 defeat at Manchester City due to a foot injury and has been ruled out of the final weekend of Premier League fixtures before the World Cup break.
Reports in Serbia claim X-rays have shown the 29-year-old has been playing with two broken bones in his foot.
Serbia fear Mitrovic will miss their World Cup opener against Brazil on November 24, but hope he will be fit for their second group match, against Cameroon, four days later.
Only Erling Haaland and Harry Kane have scored more Premier League goals than Mitrovic this season, and being without their top scorer against United is a blow for Fulham, who sit ninth in the table and are looking to continue their fine start.
In his absence, manager Marco Silva could turn to Brazilian Carlos Vinicius to lead the line at Craven Cottage.