Gilham hyper-extended his knee in a marking contest with Carlton full-forward Brendan Fevola during a NAB Cup match on Sunday.
He immediately left the ground and took no further part in the game.
On Monday, the club issued a statement saying the 24-year-old had sustained bone bruising to his femur and tibia, but he had been cleared of ligament damage.
No time frame was placed on Gilham’s recovery.
“The injury will require a few weeks to settle and will be assessed further during this time,” the club said.
Gilham will be joined on the sidelines by two of his teammates who also suffered injuries during Sunday’s costly loss to the Blues.
Rugged utility Campbell Brown has been diagnosed with a sprained ankle that is expected to keep him out of action for two to three weeks.
Grant Birchall will also miss three weeks with a hamstring injury.
The casualties are a worry for Hawthorn's backline, with Trent Croad (foot) in doubt for the beginning of the premiership season and Luke Hodge having yet to make a pre-season appearance.
