I always accepted this doubt to be by analogy. Doubiting that Chirst and be truly present in the Eucharist denies that all things are possible in Christ, which would result in doubt that he could truly come to be fully human. However, that's only part of the mystery. The other point has to do with the response of accident to substance. (Yeah, I know, anyone who knows Aquinas is going to say, "Well, Duh!") It was actually St. Thomas's analogy of this attraction in creatures:
Some sort of example can be found in creatures. Thus subject and accident are not united in such a way that some third thing is formed from them. In a union of this kind, the subject does not have the function of a part, but is an integral whole, which is a person, hypostatis, and suppositum. But the accident is drawn to the personality of the subject, so that the person of the man and of the color of whiteness is one and the same, and the hypostasis or suppositum is likewise the same.
Earlier he says, "Yet the soul and body are drawn to the personality of the divine person, so that He is the person of the Son of God and is also the person, hypostasis, and suppositum of the Son of man."
Amazing how such a simple analogy can make a mystery seem so clear without reducing its mystery.