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.
Tenshin's already thrown out a few impressive highlights in the ring since Floyd announced the tilt was back on ... and now the kickboxer is sending Money a message on social media.
The vid shows Tenshin poppin' off lightning-quick moves ... and some seriously fast jabs -- writing he's not sure how Floyd (who's 41, btw) can keep up.
Per Mayweather, there won't be any kicks involved in the scrap ... but dude's clearly gonna have to contend with some quick hands regardless.
The fight still looks like it'll go down on New Year's Eve ... can't wait!