People go on about how he won the Primary debates with bravado and yelling over people, but people seem to forget how much of the debates he didn't engage in. Like the one where the other candidates (and the Fox moderators) were very hostile to him and the first half of the debate was just them taking shots at him, the whole second half of the debate, he was very quiet and most of it was Cruz talking about other stuff unrelated to Trump. That was the only Primary debate where he was really on the defensive and he just shut down and melted into the background.
That obviously can't happen here, because it's not going to be 4-6 people on stage, it's going to be one-on-one, him staying quiet and out of the arguments if he's looking bad or just has nothing to say isn't an option. Any time he's not talking, is her talking. Him staying quiet to save face is pretty much just giving her an open platform to a third of the country to talk about her own campaign, and she'll be pressing him on answers he most likely doesn't have, anyway.