Foreman rarely took much damage if any at all. Even in the Ali fight, he didn't take many punches, and Ali was notoriously a light puncher; he collapsed due to exhaustion, he didn't have his head took off. In any other fight he took even less damage. Holyfiled hit him some, but rarely clean and Hopkins is a light puncher. There's no reason for Foreman to be slurring, unless he did wars with sparrings, but he almost surely didn't.
Foreman didn't take punishment in a lot of fights, only in a handful of fights. His fights were usually over quick because he knocked his opponent out. Let's compare some other power punchers, Tyson and Bruno don't seem to have speech or mental issues. Their mental issues stem from elsewhere, not from the punishment in the ring. They only got punished in a handful of fights, most fights they ended quickly and avoided punishment.
I think a lot of you guys are missing one all important factor here.... Nobody took liberties with big George in sparring.