Cook's lower right leg was fractured in the sixth inning Wednesday night. Rockies manager Jim Tracy confirmed the injury after a 9-2 win over the Reds, but said the injury may not end his season.
Cook hobbled around the mound as the ball ricocheted toward catcher Miguel Olivo. After the team trainer and Tracy spent several minutes with him on the mound, Cook walked off the field.
The right-hander allowed six hits and one run in five innings.
Cook was making his second start since coming off the 15-day disabled list Sept. 3. He missed 27 games with turf toe on his right foot.