Tony Bellew revealed that he personally begged David Haye, 37-years-old, to retire from the sport before he permanently damages himself.
On Saturday night, Bellew handed Haye a one-sided demolition.
Haye went down three times in the contest and eventually stopped in the fifth round at the O2 Arena in London.
In the first contest, which took place in March of 2017 at the same venue, Haye was stopped in the eleventh round. The loss was one of the biggest upsets of the year, with Bellew moving up from cruiserweight to take the fight.
But some felt the outcome was stained due to Haye suffering a severe Achilles injury in the sixth round.
After the contest was over, Haye underwent surgery and fired his coach Shane McGuigan, because the trainer advised him to retire from the sport.
Haye then retired the services of Cuban trainer Ismael Salas, works with numerous top fighters like Jorge Linares.
Despite suffering the second consecutive defeat of his career, Haye is still not willing to announce his retirement, at least not yet, and he refuses to rule out the possibility of fighting again in the future.
If Haye continues to fight, Bellew fears his former rival will get an even bigger beating by a harder punching heavyweight.
"We got up close and I banged him up," said Bellew. "He has never been out on the floor like that with shots to the chin. Wladimir Klitschko didn't even do that to him.
"His first words when he hugged me were that I'd done him. I said to him 'please stop'. This is a young man's game - a young man's game for the attributes that Haye relies on. Boxing does not favour fighters past 35 who fight on reflexes and explosive power.
"All the attributes he had over heavyweights over the years, he doesn't have any more. Don't detract from his career - he's an amazing fighter and will go down in history better than I ever will."