"Sugar" Ray Leonard thinks Conor McGregor is a very tough dude -- but when it comes to his fight with Floyd Mayweather he has one piece of advice ... "DUCK!"
The boxing legend -- who beat Floyd's dad in 1978 -- was at LAX when he told us why he believes the Irish superstar is in for a beating when they face off on August 26.
Leonard says he's impressed by the paychecks both fighters will pull in -- but he'll be even more impressed if Conor can go the distance with The Best Ever.