Centre-back Holmar Eyjolfsson, who joined on loan from West Ham and made his debut against Notts County, is away with the Iceland Under-21 side.
Allen will also be without David Bird, who picked up a calf strain in training.
Midfielder Marley Watkins, who was on the bench against Notts County following illness, could be pushing for a starting place while Josh Low should come back in for Eyjolfsson at right back.
"It's a blow to lose David Bird," Allen told his club's official website. "It's my job to get someone in to do a stint on that side now, as well as on the left hand side."
Source: Team_Talk