Don't think you know how sponsorship deals work. LeBron signed a well over 1 billion Nike lifetime deal.
Granted Lomachenko is no LeBron, but he is considered by most best p4p and is probably behind in popularity only to the likes of Canelo, and Pacman (and possibly GGG and Joshua but unlikely). He is also in his absolute prime, and will likely be active for a lot more years (unlike Pac and GGG).
He is not at superstar level pay yet, but neither was Curry 2 years ago. However he must have had a shitload of sponsor offers and if he's locked into anything exclusive or over a longer period of time that contract should be massive. Have no idea if that is the case with Reebok, but if it's not some other sponsor will step in and offer him a shitload more than he is currently making per fight. No such luck for UFC fighters unfortunately.