Scunthorpe midfielder Jimmy Ryan is waiting to learn the severity of the damage after a recurrence of his knee injury.
The 23-year-old was sidelined for nearly three months due to a cruciate ligament problem before returning to the side early last month, but is not expected to play any part in Saturday's clash at Colchester.
Knill told the club's official website: "Jimmy is still on the treatment table. We're still assessing that and I don't think it's looking too good for the weekend.
"The injury is in the same area of the knee, but we're hopeful that he'll be back a lot sooner this time than he was last."
Ryan signed for Scunthorpe from Accrington in the summer for a fee set by a tribunal that could rise to £195,000, but has been restricted to 15 appearances this season.