The Belgium worldwide sprained his left knee in the World Cup qualifier against Bosnia-Herzegovina on Saturday, as he lasted just 29 minutes of his team's 4-3 win in Sarajevo.
In-form Fellaini played for only 29 minutes before limping off on field with a knee injury. "We fear that there's ligament damage", Martinez said post match.
But he will miss United's bumper Premier League clash with Liverpool and the next two weeks.
Belgium's official national team account has published a tweet which says Fellaini will be out for a "couple of weeks", and won't be in contention for the worldwide against Cyprus.
The manager had added after the game, as quoted by Sky Sports: "We are all anxious about Fellaini".More news: Singer Jason Aldean calls Las Vegas shooting rampage as beyond horrific
Even though the Belgian ace may have avoided any major cruciate ligament damage, his sprained Medial Collateral Ligament (MCL) will give Mourinho and the Red Devils' fans a major headache.
The news is a blow to José Mourinho as United's manager is already without Paul Pogba due to a serious hamstring problem.
A resurgent Fellaini has scored three Premier League goals this season, as well as netting in the Champions League defeat of Basel at Old Trafford, after stepping up in the absence of the injured Pogba.
Fellaini has scored four goals and provided one assist this season already for Manchester United, who return to action after the global break with a trip to Liverpool in the Premier League next Saturday. The Belgian FA have since confirmed the injury which immediately rules him out of Belgium's final qualifier against Cyprus, but more importantly, Manchester United's trip to Liverpool on Saturday 14 October.