The AP is reporting that Anna Li will now be the US team alternate, after injuring her abdominal muscle in training. This is a huge bummer for the Americans on bars -- they really needed that high score from Li, and unfortunately, the world team doesn't have a comparable fill-in for her. It will have to be McKayla Maroney, Sabrina Vega, or Alexandra Raisman competing bars in the team finals, and each of them is a little weak on the event. My guess is it will be Maroney, depending on her performance in prelims. The US can still win, I think, but will have to have a great day in team finals.
Li is blogging about worlds on her official website, and it's definitely worth a read. She's honest and entertaining, and gives a great look at what the team's experience in Tokyo has been like so far.