Carl Baker will be out for around seven weeks due to an ankle injury.
Baker has scored five goals in 28 appearances in 2015/16 but now looks set to be sidelined until March.
Karl Robinson said: “We’re devastated to lose Carl for an extended period of time. He is our top goalscorer and was our Player of the Year last season – losing a player like that will have an impact on any team.
“It was great to see him in the crowd on Saturday supporting the boys. It goes without saying that we wish him all the best with his recovery.”