The midfielder suffered knee ligament damage when he made a goal-saving challenge on Reading's Shane Long during the second half of Tuesday night's 2-1 defeat at the Madejski Stadium, and could be sidelined until mid-April.
Abdou has made 31 appearances for the Lions this season and boss Kenny Jackett must now find a replacement in the centre.
Tamika Mkanadawire is likely to step up from his recent role in defence to fill the void against Nottingham Forest this weekend.
Source: PA
Source: PA