Johnson lost most of his fights against the elite of his day. Castro made him see galaxies. In Golovkin we have a boxer-puncher that can hit with the best of them. Reggie doesn't have the sort of killer power to keep Golovkin off of him, in my opinion. I don't doubt Johnson puts up a good fight, and he might see the end, but I don't see him winning.
I've got Johnson by decision over the 2017 version of Golovkin. Gennady has really slowed down a lot over the last two or three years. Although he got some fortunate stoppage wins, he had his hands full with the powerful, but limited, David Lemieux, and was getting out-boxed and outfought by smaller Kell Brook until Brook's eye socket betrayed him. Reggie Johnson was a much better middleweight than either of those men. Prime vs prime: Golovkin likely edges Johnson by decision in a close fight. Prime Golovkin decapitates 2017 Golovkin.