There was a mad scramble to protect Conor McGregor's genitals in the minutes before the Mayweather fight ... when he realized he left his protective cup at home! 

Conor had just had his hands wrapped -- while still wearing his pimped out suit -- when he realized he was missing his custom jockstrap/cup combo and his mouthpiece.

He immediately remembered he left 'em both at his Vegas home and sent one of his team members racing out of the T-Mobile Arena to retrieve his junk protection, ASAP. 

The good news ... the guy made it back with plenty of time to spare. 

But why did Conor have a custom cup? We'll just refer you back to the weigh-in ...