Oscar DelaHoya was never the same after Ike Quartey landed that lefthook that put him down hard. Oscar couldn't be that devastating slugger he once had been at lightweight. He became tentative, played it safe a few too many times.
Foreman after Ali. He was broken mentally after Ali and he was never the same for the rest of his first career.
Never the same after 1st losses Lloyd honeyghan Don Curry The above 2 looked like imposters before & during their 1st real losses ( Starling in Lloyd's case) and than never the same The below 2 fought as themselves got beat than were never the same Jeff Fenech Michael Nunn
Just had a conversation where an Errol Spence fan insisted that Kell Brook was never the same after fighting Spence. I'd say it was more the GGG fight that ruined Brook. GGG beat Brook when he was undefeated, and damaged his eye socket. Spence fought Brook well, but Brook was actually beating Spence for the first 5 or 6 rounds. GGG ruined Brook, not Spence.
That was the OP’s thesis, but I think it can work a lot of ways. For instance, Marvelous Marvin Hagler was never the same after his draw with Vito Antuofermo in his first attempt to win the title. While many thought he deserved to win, he left a lot on the table and enough doubt that it could be scored a draw — and he became a man possessed after that. So he was never the same after that draw. Jimmy Ellis was never the same after losing to Don Fulmer and George Benton, back-to-back, by split and majority decision at middleweight in late 1964. Ellis had two transition fights around light heavyweight the next year and by ‘66 had transformed into a heavyweight - tested the waters with a handful of fights in 1966 and started taking apart contenders and promising prospects thereafter on his way to laying claim to the WBA title. Mike Weaver was never the same after losing to Larry Holmes in one of the most action-packed heavyweight title fights. He was an unknown contender who had started to turn things around, but after establishing himself in the Holmes challenge he was a staple in the heavyweight scene — ranked in the top 10 in the Ring’s annual ratings for 7 of the next 8 years, including two years at No. 1 (behind Holmes, who was recognized as champ).
I'd agree with that. The Golovkin bout left an impression that Brook would carry with him. I was at the Brook/Spence bout and there was a distinct sense that Brook was drifting out of the fight, some rounds before it actually came to an end. Spence fought a solid, methodical fight (as seems to be his way) to take the initiative and claim the title. Fair play to him, but he wasn't dishing out career-changing punishment, in my opinion.