Firstly, Floyd has longer arms than Sergio. 26" arm length vs 24"
This thread is saturated with dumb motherfuckers who don't even talk boxing, but I've come to accept that as part of esb.
Sergio has yet to fight a single HOFer in his career, but has shown he doesn't always have all the answers. Floyd has looked better than Sergio has throughout their careers despite facing more skilled opposition.
I don't know where all these ideas of Sergio stopping Floyd easy comes from. Sergio has said on many occasions that Floyd would be the hardest fight.
It isn't going to be no cakewalk for Sergio if he has to face someone with these skills and athleticism
I think Floyd would start off in his shell guard.
Macklin guarded like that, and it protected him against left hands while he managed to make it last.
While in this guard, the focus is investing early to the body with str8 rights. The guard itself makes it hard for Sergio to land his left downstairs. Do bodywork to take away Sergio's primary defensive instrument, his legs.
Floyd can switch it up to the high guard later in the fight. Counter Sergio's jab with his own jab from this position due to where Sergio likes to return his lead hand after jabbing.
It won't be hard for Floyd to land lead rights onto Sergio's face, especially once Sergio's movement is chopped down from the bodyshots. Floyd is great at throwing short punches, and Sergio's hands down style is a liability against a compact offense. Barker's shorter punches didn't give Sergio much to counter off of. The last hands down opponent Floyd fought was Zhen, and he had no problem connecting with lead rights.
Floyd's low output style will bother Sergio since he's less effective leading than as he is countering. Macklin and Barker had success by being offensively conservative which didn't give Sergio much to work off of.