I feel like this is a bad turning of events from the authors. There are 3 possible scenarios here:
1) He recovers completely and miraculously. At that point, the author led us to believe that something was objectively impossible (medical opinions and all that), but instead it worked.
2) He learn something from the old trainer of his mentor, just to preserve stamina or something like that (Which is equally implausible and would be like watching an entire comic book about Jun Misugi from Captain Tsubasa)
3) He retires and we have another Hajime no Ippo situation
I feel like i wouldn't like (not equally though) all three options
