Question: Wasn't Rocky supposed to have a brain injury from Rocky V, that prevents him from fighting?
Answer: This is true, however it was something invented for Rocky V that was supposed to be from Rocky IV that was only introduced into V after. V is considered to be the worst film of the series with very poor reception. Stallone himself has even gone on record saying that he hates V. So this sixth film almost pretends that film never happened. V is still technically canon with this film, however six basically ignores a lot of problem with that film and tries to recton it. Does this create inconsistencies and continuity issues with the series? yes. Is the film better for it? Definitely. So where as that was supposed to be the case after V, they pretty much said with this film that no... we are not doing that any more pretend V didn't happen... but it did happen. It's a soft reboot in a way.
Answer: Stallone himself answered this question in an interview. "When Rocky was diagnosed with brain damage he never went for a second opinion and yielded to his wife's wishes to stop. So with the advent of new research techniques into brain damage, Rocky was found to be normal among fighters and he was suffering the results of a severe concussion. By today's standards Rocky Balboa would be given a clean bill of health for fighters.