Jake appeared on "In Depth With Graham Besinger," and talked about something boxers rarely discuss when they're still in the game -- how boxing messes up their head.

Jake told Graham, "I notice it in conversations with like, with my girlfriend or friends, like, not remembering something that I should be able to remember that happened a couple days ago."

He went on ... "Sometimes in my speech, where like every hundredth or two hundredth word, I'll mess up or, like, slur, which I didn't do that before."

Jake says his problems actually started with football, but are clearly worsened by boxing. He doesn't even remember how many concussions he's suffered -- somewhere between 20 and 30.

Again, something boxers don't say ... Jake says he's scared about the long-term effects, although he thinks certain psychedelic drugs could help abate the problems.

It's pretty shocking he's saying all this when he's about to square off with former UFC Champ Tyron Woodley Saturday night. He beat Tyron by split decision in the past and the rematch promises to be fierce.

Jake said his doctor was blunt -- stay away from the ring, but that's not gonna happen.