Should he be out for two weeks he will miss next weekend's Premier League trip to Liverpool.
Mourinho had bemoaned the loss of the lanky Belgian for his side's victory in Russian Federation, and won't be too pleased with the news of his latest injury ahead of next weekend's big clash at Anfield.
"We are all anxious about Fellaini", Martinez was quoted as saying by Sky Sports.
Despite sitting out of training for much of the week, Fellaini was cleared to play in Saturday's (7 October) penultimate 2018 World Cup qualifier against Bosnia and Herzegovina by manager Roberto Martinez.
"We fear that there's ligament damage."A Belgian FA official also confirmed the injury, and said: "It is something to be anxious about on first sight".
Fellaini is now officially ruled out of Belgium's clash against Cyprus on Tuesday with a medial collateral ligament strain to his left knee. Fellow Man U player Romelu Lukaku was rested by the side for the match after picking up an ankle injury.
His injury will undoubtedly cause Jose Mourinho a headache. The Premier League champions are at Crystal Palace on Saturday.... Though often derided, Fellaini has experienced a bit of a resurgence in recent times, having scored four goals for his club this season.