The 29-year-old has impressed for United this season and in United's last game before the worldwide break, he scored twice as they beat Crystal Palace 4-0.
However, the towering Belgian faces will definitely miss Saturday's clash with Liverpool and possibly longer, depending on the results of a scan.
Speaking ahead of the match, Mourinho ruled Fellaini out of the match.
Syrian FM says U.S. coalition sparing IS in Syria
In late September, the coalition acknowledged the deaths of 735 civilians in its strikes on Syria and Iraq since 2014, but the Britain-based Observatory says hundreds have been killed in Raqqa alone since June.
"You can say, like some people do, we have not played one of the top five teams but last season we did not beat West Ham at home, last season we did not beat Everton at home or Southampton away".
Mourinho said: 'We may cry with the process that leads to injuries but we do not cry because of injuries'.
Mourinho's United have been imperious in the Premier League so far this season, scoring 21 goals and conceding just twice. 'At Anfield I've had great moments and I've had bad moments, ' he told Sky Sports. Only this is not about switching Romelu Lukaku into Liverpool's line-up to cure their profligacy, or drafting Mohamed Salah into Manchester United's team to add pace. Does that mean we are going to be as successful as last season, winning two titles?
Sensibly, Martinez only sent on the 24-year-old with just 25 minutes to go, although the need to do so was questionable given that they were 3-0 up over the Cypriots.