Boxing star Jermell Charlo is REALLY confident his guy Errol Spence Jr. will handle business and remain undefeated on Saturday ... telling TMZ Sports he's putting a $25,000 bet on The Big Fish to beat Terence Crawford. We also spoke with former boxer Shawn Porter -- who fought both Bud AND Errol during his career ... and he weighed in on what he's calling "The Super Bowl of Boxing."