The 24-year old striker sustained a hamstring injury in Monday's 3-0 win over Motherwell with scans revealing a serious tear which will see him out for at least six weeks according to manager Ally McCoist.
"It's not particularly good news for Kyle. He looks to have a tear in the hamstring. At this moment he is on crutches and we think probably six or seven weeks I'm afraid," McCoist said.
In further injury news to 'Gers strikers, Nikica Jelavic is likely to miss Sunday's Scottish Cup trip to second division Arbroath.
"Jelavic is a bit better. I spoke with him and the groin injury is not as bad as it was. It's too early to say whether he will be available for the Cup tie," McCoist added.