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This is a very interesting scenario, but Bivol has doctors and physicians, and they will not let him go to full training and agreed to fight Callum, who is still a dangerous and capable challenger to any champion.
I expect Bivol to be in perfect health going into this fight, so I'll go with Bivol to win; this is good preparation for a possible Benavidez match, who is fresh from beating Ramirez, or a possible third match against Beterbiev. Whatever, Bivol is still a capable champion, and hopefully we'll see him fight Benavidez or Beterbiev.
I expect Bivol to win, but with his past surgery, and how he fought in his last fight, I believe that he's still not in 100%. Like what you said though, he has professional doctors in his team, and if they say that he can't then vacating the title is the best option for him.
I did some research, and it says there that Bivol has FULLY recovered already on his surgery, so maybe we could expect him to fight in this match. Now is the time for Bivol to say to the other fighters that he's back and he's ready from any fighters that will try and get those 3 belts from him. Like what you said, there's also a chance that Benavidez could come down again and fight him after this fight probably at the end of the year or next year. Who knows. As for a trilogy between him and Beterbiev, I'm thinking if Beterbiev can still fight because it's more than a year since his last fight.
Anyway, this will be an interesting fight. A former champion vs. a current champion.