That won't be an issue for him, the primary debates were basically "Trump vs. everyone else" anyways. He loves to talk.
Will he have anything substantive to say that he and his campaign/surrogates won't be backpedaling on within 24 hours is the bigger question.
Sure, if you want to end up like Rubio repeating the exact same phrase during one of the primary debates and short circuiting, there's more than just memorization and repetition. Yes, they need to hammer their core messaging through, but knowing how to tailor responses to specific questions (or how to redirect effectively) takes more than just memorizing a handful of sound bites.
So either Trump is incapable of remembering that information or has chosen not to bother trying so far, because thus far this task that you present as so simple has been well out of reach of Trump throughout his campaign : P