Idk if you can really consider Pavlik because he moved up in weight against Hopkins and performed similar to most of Hopkins opponents with that flat footed style. But after he dominated Rubio and Espino before being defeated by Sergio which again someone who’s a mover with fast hands.. I just feel that those were more of bad matchups for him than a decline.. I’m gonna go with Sergio from JCC Jr to the Macklin. When he fought Macklin due to his legs being completely **** due to age and injuries he just couldn’t throw those shots from the hip as he once did.. And Paul William from the Sergio fights to being completely shot in the next fight with Lara..
Also Kovalev Yarde to Kovalev Canelo. From past it to completely shot. Vs Canelo he looked like David Price.
He looked physically weak. I know they changed up his style to focus less on power, which was a smart idea as he could still box anyone's eyes off. But he was just physically weak and those pity patty shots made it look even worse. So weird seeing someone still be able to box at a high level while their body has just given up.
Lewis-Tyson to Lewis-Vitali? Though of course the quality of opponent was rather different, Tyson being totally shot [18 years ago, never mind now] & Vitali improving. That's in terms of a decline with an age component. Declines that were more burger/taco based would be Douglas/Ruiz following their title wins. Golota-Lewis after Golota-Bowe was very different, but again there was a step up in quality there.
Supposedly RJJ. Don't want to elaborate on the "supposedly" as I've been talking **** about Roy a bit too much. I'll just say I think it was more Tarver than anything.