The 29-year-old suffered the injury in Belgium's 4-3 victory over Bosnia-Herzegovina in Saturday's World Cup qualifier at Stadion Grbavica. Roberto Martinez, the Belgium coach, said he feared ligament damage.
United will be hoping for positive news on the midfielder ahead of their Premier League clash against arch rivals Liverpool on October 14.
The Belgian has enjoyed a rich vein of form lately, scoring four goals in all competitions and adding a further assist for Jose Mourinho's side this season. Mourinho confirmed that Pogba is a "long-term" absentee with the injury worse than first feared, but the Frenchman has shown considerable progress by stepping up his recovery process.
It's claimed Fellaini will be out for a "couple of weeks" after the scan confirmed a "MCL sprain". Get well soon Marouane!'
Belgium manager and utter fraud Roberto Martínez did not sound hopeful: "We are all anxious about Fellaini".
Fellaini is now officially ruled out of Belgium's clash against Cyprus on Tuesday with a medial collateral ligament strain to his left knee.
Fellaini has impressed during his stint in central midfield this season and scored twice in the 4-0 win over Crystal Palace before the worldwide break.
He looks set to join fellow United midfielder Paul Pogba, who is out with a hamstring injury, on the sidelines for his club.
United may breathe a slight sigh of a relief given their midfielder escaped without cruciate ligament damage which would have extended his time in the treatment room from weeks to months.
Fellaini could miss seven games in just three weeks.