According to Fighters Only:
Steve Cantwell is off the UFC 120 card less than 48 hours before it is due to start, having ripped a knee ligament in a BJJ training session on Thursday night (UK time).
Fighters Only was told on Friday morning that Cantwell has consulted medical staff but is clearly in no condition to compete and has had to be pulled from the bout with UFC debutant Stanislav Nedkov.
He cannot be replaced at this late stage; as there would be no time for a replacement to pass medical checks.