I'm not sure Kostya could win this anymore... He would need all his younger reflexes to replicate what he did 4 years ago IMO if Judah keeps calm the whole fight, and knows that he can't take Kostya out with one punch. If Mayweather and Judah are still friends and Mayweather trains with Judah I think He would easily beat Kostya, but I wouldn't bet on Mayweather willing to be a lead sparring partner for somebody else...