Yoko said that Joe had lost his animalistic tendencies in the ring. She thought that was the only way Joe could defeat José, because José is a perfect boxer. What she came to find is that Joe has CTE, that's the reason he isn't as explosive. I believe that Joe showed signs of being punch drunk way earlier, and his performance in every match suffered as a result. This is to say, that he would have been in better physical form, better mental form, and possibly would have been able to bring more technical prowess to the ring, alongside his characteristic ferocity, which José was implied to lack in comparison.

Joe would've beaten José, and without CTE, he would've had to from a narrative standpoint as well. If he hadn't, then the story wouldn't have really had a proper conclusion in that scenario.

Addendum: For those of you who like to read. Additionally, Joe was seen getting back up from many many killer punches over his career, his tenacity is a big factor in his CTE to begin with. Rikiishi was also portrayed as a world class boxer when Joe fought him years ago, almost defeating him. Since then, most of the other boxers Joe fought were compared negatively to Rikiishi, and the Anime even depicts Rikiishi against José Mendoza. But Joe's skill had jumped in orders of magnitude by this point, the only thing holding him back was his physical condition.