The expectations for him are so ludicrously low, and so high for Hillary, that normally a "debate flop" in any other year would probably mean almost nothing to him this year. He's constantly selling a narrative to his followers about unfair media and rigged debates. It only counts if he "wins".
He's running behind Hillary just about everywhere that counts, and his demographics are worse than Romney or McCain ever were. He can sell that narrative to his followers and declare himself the King of Debates all he wants but he's got to make gains somewhere. The debates are a much wider audience and he needs to look presidential to the people he needs, college educated whites (since it's a foregone conclusion he won't pick up any minority groups). For that to happen he has to keep his dick in his pants (I shouldn't have to say so, but I mean both figuratively AND literally) and look at least slightly respectable. That's the only way he really wins anything tomorrow night, regardless of who he or the pundits declare the victor.