I can't speak to percentages, but Trump holds a tenuous grip on a not insignificant portion of undecideds and the more principled members of Republican base. If he comes across as a crass, completely ignorant doofus, it could convince some of them to abstain from voting for him. He may not go down in flames, but it could tip a few battleground states more blue.
I think the low expectations of him in the debate is a double edged sword. If he stay's above water that is decent, but Hillary will still look better than him depending on what she does. Staying calm, defending herself properly, and introducing herself right will make her look better even if he does survive. If people expect him to do that then their opinion won't necessary change towards either candidate much, besides some final having an excuse to back one or the other. I see it that Trump needs to do far more than having his expectations being met, he needs to far exceed them.
The thing that Donald Trump has over Clinton is dominating his opponents with a little more than bluster and putting his opponents down. His opponents usually don't go on the assault because of that. However, anything besides that he crumbles and can't keep up because he does not know what he is talking about. It makes him look really stupid, and if he is under pressure it looks like he can't handle himself. The lying isn't really the biggest problem he has, but is that fact that a lot things he can say is very vague and lacking in detail; very easy to attack and some the things he proposals; very easy to attack. Hillary has the moral high ground just from being his adversary she can really easily take a strong stance against his ideas and very easily tare them apart. She'll score easy points just from doing that.
His bluster and taunts might not work against Hillary and it seems that it never did, but sometimes it did look like she looses her composure from irritation from certain questions, however it looks like she is preparing for that. Hillary has a lot to use against Trump, the main problem is utilizing it at the right time since he can be pretty unpredictable.