I have several friends who have hurt (as in broken and bruised) their ribs, and it hurts. A lot. Especially on the day after it occured. And I am also a singer, and let me tell you that singing (especially belting) requires a lot of work from the diaphram and ribs, which means that she would be placing a lot of stress on a recently injured part of her body. Health wise, she should not go on. Then she has to run around acting (as someone wisely pointed out), which will hurt her ribs some more. And, ON TOP OF ALL THIS, she gets to go to Rent this week and start working immediatly on the songs, because they are going to be recording the music soon. So, realistically, she is not going to be on tomorrow.
And we should not forget shoshana bean, who must be feeling awkward and horrible about taking over for Idina on her big night. So my best wishes to Idina AND Shoshana, may tomorrow go well (and may the fans be understanding), and I hope for a speedy recovery for Idina and a great run for Shoshana.
"If you can talk, you can sing...if you can walk, you can dance."
- T.K. Greene