Any fighter can lose to a peer on their worst day. Boxers are the greatest athletes in the world in large part because 1 slight mistake can be the difference between winning or getting your head caved in. Tyson had a defense. His issue was high lack of reach and height, having to work around a good jabber and then because he was a heavyweight, having to punch up on guys. Sure, his punching power was incredible and he could overcome that punching up...but once he fought boxers that were focused on jabbing him instead of fearing his awesome punching power, he was no longer elite.
Michael Moorer was embarrassing a 49-year old George Foreman, made a mistake and got knocked out. Sugar Ray Robinson was quicker with better hand speed and had a sizeable reach advantage and lost to Carmen Basilio. Roy Jones Jr. dominated Antonio Tarver and 6 months later RJJ dominated Tarver in the first round and then was knocked out in the 2nd.
Floyd is the superior fighter to Manny, but a fighter having his worst day against a formidable opponent is going to lose. The great fighters just don't have their worst days very often.
YR