Lee McCulloch could miss the remainder of Rangers' quadruple bid after suffering a broken foot against Celtic.
The Scotland international sustained the injury in the opening stages of Wednesday night's defeat at Celtic Park when he was challenged by Paul Hartley, with the Bhoys man shown a yellow card.
McCulloch played on for the remainder of the game, but a scan has revealed the problem, with the injury now requiring further assessment.
Manager Walter Smith told the Daily Record: "The initial X-rays have shown there is a crack in Lee's foot but we must wait for the swelling to go down before it can be properly assessed.
"Even with a fracture it can sometimes be possible for a player to continue playing but we will have to wait to see what the doctors say when he returns for another scan."
McCulloch is certain to miss Sunday's Scottish Cup semi-final against St Johnstone, and goalkeeper Allan McGregor must also sit that one out due to the ankle injury which forced him off against Celtic.
McGregor could also miss the final Old Firm clash of the season on 27th April, plus both legs of the Uefa Cup semi-final against Fiorentina.
Defender David Weir is suspended for the Hampden clash against St Johnstone, having been reported for violent conduct at the end of the Old Firm clash.
And his defensive colleague Carlos Cuellar must miss the Celtic Park repeat in nine days' time after his red card for handball on the line in Wednesday's fiery derby.