Tenshin Nasukawa -- Floyd Mayweather's opponent in the upcoming Tokyo fight -- just sent a message to the boxing legend with his fists ... saying, "I’m not sure if a 42 year old can react to these."
20-year-old Tenshin's an up-and-coming star in Japan -- one RIZIN fighter told TMZ Sports he's the Michael Jordan of kickboxing over there -- but Floyd is set to throw down with him anyway next month.
The fight's only supposed to be an exhibition -- per Floyd, "I'm moving around with the guy for 9 minutes" -- but, clearly, Nasukawa ain't takin' the bout lightly.