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 is a trusted lieutenant of manager, Jose Mourinho who has praised his work rate and willingness to put in the hard shift for the team.
Belgium however went on to win the match 4-3 having already qualified for 2018 FIFA World Cup.
Fellaini was due to be scanned on Sunday morning, and this outcome would fit in with that timeline. That's the good news.
While no injury setback is welcomed, fears were that Fellaini could be out for months with his knee problem.
Fellaini is now a doubt for next weekend's match away at Liverpool, and will possibly be unavailable for weeks after that.
'Scan - left knee injury (MCL sprain).
Manchester United chiefs are convinced Marouane Fellaini will soon agree terms over a new contract.
Ander Herrera looks set to partner Nemanja Matic in midfield, with Daley Blind and Juan Mata among the other alternatives to play in the middle of the park.
His injury will undoubtedly cause Jose Mourinho a headache. More worryingly for United, the injury comes at a time when European and domestic fixtures are starting to come thick and fast.
Fellaini sat out of training with his national team this week, owing to an Achilles injury picked up following a nasty challenge from Southampton's Shane Long on 23 September.