Manabu wins mid-high diff for several reasons. His AP will demolish Johnny, as well as his intelligence and grappling skills which will punish Johnny's temper and aggressive fighting style. Karate teaches grappling but Manabu practically mastered aikido, so even if Johnny tries to use his strength advantage to overwhelm him, he won't win when it comes to any form of grappling.
Now Johnny might choose to play dirty and take advantage of the fact that Manabu wears glasses. He'll likely get cocky and overconfident, only to get countered and hit, or get grappled instead. Ryueen is a master of using dirty tactics and has hundreds, likely thousands of street fight experience. However, when Ryueen tried to use these cheap tactics against Manabu, such as covering his glasses with snow and attacking his weak point, Manabu still countered him, with his EYES CLOSED.
Manabu slams with more AP and dura, more Stamina, higher intelligence, and having the skills necessary to counter anything Johnny might do.