Well we don't know, we were first told Rotator Cuff for the JDS fight and then last week I heard it was an ACL issue. When I hear ACL from a wrestler my immediate reaction to take 5 years off their career.
It was a load of rubbish. He picked up a minor injury in training, like most fighters go through in preparation for all of their fights. It was the huge shoulder injury during the Brock fight which contributed to Cain looking so "off" in the JDS fight.