Ali-Foreman Could the champ win again?

I've been a fan of The Greatest since the start of his "second career" I was a little too young to really remember Cius Clay. However,the one thing that disapoints me is Ali never gave George Foreman a rematch. He talked about it, even right after his third fight with Joe Frazier. But maybe its just as well, since I doubt if he could have beaten George a second time.(especially after Thrilla in Manila) Any historians care to weigh in?