Rob Jones and Cliff Byrne have joined Eddie Nolan on the injury list at Glanford Park.
Nolan suffered a sprained ankle during Tuesday's 1-0 loss to York City and will not befit for next week's Championship opener at Reading.
Byrne suffered a hip problem in training on Thursday and missed Saturday's friendly against Huddersfield.
Jones, meanwhile, started against the Terriers but had to be replaced after 10 minutes because of a groin problem.
Adkins said it was too soon to tell if Jones would be able to face the Royals next week, but expected Byrne to be available.