Rangers say striker Martyn Waghorn will be out for a "number of weeks" but does not need surgery on his injured knee.
The 26-year-old went off immediately after winning and converting a fourth-minute penalty in their William Hill Scottish Cup win over Kilmarnock.
Waghorn went down on the artificial surface after getting in front of Killie left-back Steven Smith and left Rugby Park on crutches.
A Rangers statement read: "The knee injury Martyn Waghorn sustained in Tuesday night's Scottish Cup win over Kilmarnock is not as bad as was first feared.
"Martyn suffered the injury in the challenge that saw Rangers win their early penalty and although he recovered enough to score from the spot, he could not continue.
"The initial impact on an unforgiving Rugby Park surface resulted in bruising and lacerations which were immediately visible.
"However, thankfully the scan has revealed there will be no need for surgery, although Martyn will be sidelined for a number of weeks.
"His rehabilitation will be expertly managed by the medical team and we look forward to welcoming him back sooner rather than later."
The former Sunderland and Wigan striker has scored 28 goals for Rangers this season. His replacement, Nicky Clark, hit an injury-time winner at Rugby Park.