In the trailer Mary Anne calls Adonis out on apparently using Apollo as an excuse for fighting Victor... and she might be right. Adonis has never wanted to just live up to Apollo's image; he wanted to surpass it. What better way than taking on and defeating the son of the same fighter that his father not only couldnt beat, but was killed by?
It's mentioned early on that Viktor fights with "an unorthodox style" in spite of everyone knowing how Ivan fought. In fact, everything about Viktor's style and tactics make sense when you realize how he was trained:
Since the Dragos now live in poverty and don't have access to the facilities and Soviet Super Science that made Ivan nearly superhuman, Viktor has built up more muscle than his father in part because he has to work for a living and can't afford more aerobic activity in his schedule... Or the steroids his father was injected with. He's compensated with (even more) raw power for the inevitable lack of endurance he's just not built to have.
Viktor repeatedly uses body strikes to set up his head shots on Donnie, explicitly at his father's instruction. Of course he does! Ivan learned the hard way how punishing repeated body shots are to even the strongest fighters, since Rocky "chopped him down" in their fight. He just took the tactic that worked on himself, and applies it to his son's opponents!
Adding to the above, Ivan learned what it was like to be largely on his own at the end of Rocky IV, and has chosen to ensure that his son never experiences that, as well as supplying enough psychological warfare and game planning that his son can focus on his job and adjust during a fight, instead of sticking with a single strategy as Ivan was forced to against Rocky.
The result? A Lightning Bruiser of a slugger who fights smart, has a strong pre-fight psychology attack run by his father, and who can't be beaten the way his father was, striking at clear objectives designed to hasten the end of a fight... But still susceptible to the classic slugger weakness of endurance issues.
Rocky's fear of Donnie fighting Viktor and his line of "He broke things in me that ain't never been fixed" makes more sense if you (choose to) remember the beginning of Rocky V. Immediately after his fight with Ivan, he's already showing signs of brain trauma, something that's never been seen after any of his fights. Then he discovers his accountant screwed him over and cost him all the money he's earned throughout his career, and cost his family their home. A simple solution would be to have a few fights to win back some of it and keep his wife and son in the home and lifestyle they're accustomed to. However, he can't because the damage from the fight with Ivan forces him to retire and forces his family to return to Paulie's house and from the upper class to the lower class. Fighting Ivan Drago cost him the ability to provide for his family and he doesn't want to see the same thing happen to Donnie.
The Barclays Center in Brooklyn hosting the match between Adonis and Viktor seems a bit odd given that its hosted few high profile boxing events while nearby Madison Square Garden in Manhattan is a much more prestigious venue with a long history of hosting title boxing fights. However, given that the borough of Brooklyn has both a significant African-American (like Adonis) and Russian/Eastern European immigrant population (like Viktor), it makes sense the promoters would host the event there to draw interest and attendance from those groups.
Rockys game plan for retraining Adonis to defeat Viktor in the second fight is risky, but the smartest plan for exploiting Viktors weaknesses.
Viktor may be a Slugger-style boxer at heart, but his fathers trained him to fight smart, more similar to a Puncher-Boxer, a notorious hybrid of styles that seeks to control the fight with careful maneuvering and overwhelming power.
Adoniss regular style, Boxer, is simply going to be overwhelmed because Viktor makes few mistakes, has good defense, and way too much power.
So Rocky has Adonis mix in elements of the Swarmer style of boxing, which is the natural counter to the Boxer part of Viktors style... but vulnerable to the power of a Slugger (the Puncher part of the Viktors style.)
Rocky and Adoniss gameplan cant address Viktors full power, but it throws him off his game long enough to make it a war of attrition, where if Donnie can survive the punches that *will* connect, hell still be standing long after Viktors gameplan has collapsed from exhaustion trying to keep up.
After their first match, Viktor left Adonis with broken ribs and a ruptured kidney, peeing blood, in a wheelchair for a couple months, and needing help with many tasks for longer. All things considered, especially seeing what his father did to Rocky, Donnie got off easy. What if he was as brutal, or even more so, than Ivan?
Rocky's grandson doesn't recognize him when he opens the door for him and his son is clearly possessed of mixed feelings regarding his dad. Despite their apparent reconciliation in Rocky Balboa, it's clear that Rocky Junior still holds a lot of anger at his father.