Larry Merchant -- I'm Not Watching Floyd Fight Again ... Unless It's GGG

Wanna know what Larry Merchant WON'T be doing on Saturday night?

Watching Floyd Mayweather fight Andre Berto ... 'cause the legendary boxing commentator says that any Mayweather fight that doesn't involve GGG is a total waste of time.

Merchant was out in L.A. recently when we, of course, had to ask for his expert opinion on the huge fight (with the not so huge buzz) in Vegas this weekend.

That's when Merchant WENT IN on Floyd ... saying he won't watch because Berto is not a "serious opponent," saying that Floyd is afraid to fight the guy everyone wants to see him battle ... GGG.

BTW, it's not like Gennady Golovkin is afraid to fight Floyd -- in fact, he recently appeared on "TMZ Hollywood Sports" and called out the champ ... saying he'd beat Floyd's ass AND put on a show!

Check out the clip -- and remember that if this were 50 years ago, Larry would kick Floyd's ass himself.