Riley came off with a hamstring injury in the 4-0 defeat at Stevenage on Saturday and has not recovered in time.
Midfielder David Bird, who was playing at right-back at the weekend, also came off after suffering a knock but there is a chance he will be available.
On-loan Blackpool defender Ashley Eastham will come into the squad, but fellow loanee Medy Elito is struggling with illness and the winger is rated as doubtful.
Centre-back Andy Gallinagh came off the bench at the weekend for his first appearance since a double hernia operation and is in contention once again.
Winger Josh Low also featured as a substitute on Saturday after recovering from a thigh problem and will be pushing for a starting role.
Source: PA
Source: PA