However, on Sunday the Belgian FA announced that Fellaini will only be out a couple weeks. Get well soon Marouane!' There are fears that he has suffered knee ligament damage.
"We are all anxious about Fellaini", Martinez told reporters.
"We fear that there is ligament damage".
As well as that, the Belgian will be unavailable for United's two Champions League games against Portuguese side Benfica, and a trip to Swansea City in the EFL Cup.
Manchester United is set to visit rivals Liverpool in the eight week of English Premier League at Anfield Stadium on Saturday (14/10/2017) afternoon local time.
The 29-year-old went off after 29 minutes of a World Cup qualifying win over Bosnia-Herzegovina on Saturday.
The news is a blow to José Mourinho as United's manager is already without Paul Pogba due to a serious hamstring problem. "Ligaments, left knee. More news tomorrow".
However, Mourinho still has a fairly good replacement in the form of Ander Herrera, but loosing Fellaini, who is in the form of his life, is a setback that would be very hard to cope up with, even if it is just for a couple of weeks.
Marouane Fellaini has scored four goal for Manchester United in current season of Premier League.
A potential added injury to the growing list could not come at a worse time for United, who return after the worldwide break for a crunch fixture with bitter rivals Liverpool. Now with the absence Fellaini, MU has only three defensive midfielders, Nemanja Matic and Ander Herrera, plus captain Michael Carrick.