Gamespot has had a copy of Fight Night Champion for a couple of weeks. They've just posted a Q&A with some of their readers.
"Did they fix the problem that no matter how late in the fight or how little health you have left if you get knocked down only once you will be able to get up almost instantly? Also are the flash knockdowns completely random like last time or are they actually based on how hard the punch was thrown?
From what I've seen so far, if your stamina is still pretty high, you won't have a problem getting up after the first knockdown. But if you get knocked down in a late round or a second and third time in separate rounds, it becomes much harder to get back up quickly. As for flash knockdowns--the ones I've seen require a pretty clean, strong punch, but they still seem pretty random. Still, you probably won't knock anyone out with a random jab."