Fellaini started in midfield for Belgium but lasted just 29 minutes as he was forced off with a knee problem.
And speaking after Belgium’s 4-3 win over Bosnia, manager Martinez was unable to give a positive outlook on Fellaini’s problem.
Martinez said: ”We are all worried about Fellaini. We fear that there’s ligament damage.”
The news will come as a major blow to Manchester United boss Jose Mourinho with a huge clash against Liverpool coming next Saturday.
Fellaini was already deputising for the injured Paul Pogba, who is not expected to return until November.
He’s become a key player for Mourinho, scoring twice in the 4-0 win over Crystal Palace last weekend.
It could mean a recall for Ander Herrera, who has played a bit-part role this season.
Mourinho appears to have got his way with Romelu Lukaku, however, with Martinez resting the striker for Belgium’s dead rubber.
Lukaku will return to United with an ankle injury that will be assessed.
As for Fellaini, he did not leave the stadium on crutches and he is yet to have a scan.
