American Di Bella is the former boss of HBO TV's boxing set up and boasts 70 fighters on his books, including WBC heavyweight champion Deontay Wilder.
But Di Bella says he's excited at the addition of the 27-year-old, who will now be his only other heavyweight fighter besides Wilder.
"Junior has tremendous tools, size, reach - he has power and he has amateur pedigree," he said.
"He has to be developed properly and that's going to be the job for myself and his management to make sure that he is."