The Scotland international limped out of the Boxing Day defeat to Reading with a calf strain and joins captain Louis Carey (illness) and Liam Fontaine (ankle) on the sidelines.
Lewin Nyatanga replaced McAllister early in the second half at the Madejski Stadium and he is expected to retain his place on the left of a back four that includes fellow youngsters Christian Ribeiro and Steve Caulker, leaving Damion Stewart as the only experienced defender.
Midfielders Jamel Campbell-Ryce and Kalifa Cisse and strikers Danny Haynes and David Clarkson are available for recalls should boss Keith Millen feel the need to make further changes.
Source: PA
Source: PA