Fellaini has been with Belgium during the international break but had to leave the field after 29 minutes of the World Cup qualifier with Bosnia and Herzegovina.
The Red Devils went on to win the clash 4-3 on Saturday but Fellaini suffered a knee ligament sprain and will not be available for their final qualifier with Cyprus on Tuesday.
The midfielder was taken for tests on Saturday and Belgium confirmed that he would be sidelined for two weeks via their official Twitter account.
United boss Jose Mourinho would likely have started Fellaini for Saturday's trip to Anfield, however, he may now turn to Ander Herrera to fill in, while Michael Carrick could also come back into the fold.
Paul Pogba is still absent with a hamstring injury and Liverpool will be hoping to take advantage of a weakened middle third at the weekend.
Fellaini is also likely to miss the Champions League clash with Benfica at Old Trafford as well as United's visit to Huddersfield on October 21.