Injuries are a part of this sport, like many. It's not fair to shun the winner if the other man can't make it, but I get what you mean.
Martinez's footwork and ring movement will look very good the first part of the fight. Once Cotto's body work starts to slow him down he will feign a bad knee!!! I see that coming!!!
Martinez' knee will be fine. Like already stated these are common sports injuries and serious athletes rehab from them quite efficiently these days.