One thing I've noticed that's different with a ballet company (or at least this one) is the SM does nothing if a dancer is injured. I saw someone sprain their ankle during final dress (with an audience) and the maestro briefly paused to give two other dancers time to get her off stage and into the arms of the PT. Then, without the SM saying a word to facilitate this, another dancer immediately stepped on stage and finished the solo instead. This allowed everyone to move on with the show (and the cues remained unchanged), and the new dancer received a modest round of applause. I was in awe of how normal all of this seemed to everyone!
Not entirely true...perhaps it is in that company. Because dance is different than theater and the artistic director/choreographer stays through the run of the production (which is 1 to 3 weeks), it is up to artistic to correct the casting. When a dancer gets injured, while I'm calling the show, I text artistic about the problem and what they want done. I have time stamps in my cue sheet to say when that role needs to re-enter, my staff notifies wardrobe and hair and we wait for artistic. If artistic doesn't respond, I make the decision. And I always fill out Workers' Comp paperwork for injuries.
My very first season, as an ASM, a dancer came offstage injured. I helped him get to the floor, got him a pillow and we paged for the doctor. I knew that he had to go out and hold a maypole for the finale and I told another dancer, who was in the wrong costume (he had been on earlier in the piece) what to do and he did it. It may have looked odd visually, but that's what happens.
And maybe my company is the odd one, but I always take care of my injured dancers. I had a dancer dislocate his hip during a very contortionistic piece, which ended in a black out. It was my job to tell the PSM when he was standing for bows, and when he didn't, I told the PSM to call in the main and I ran out and carried the dancer offstage.
But, yes, dance companies have multiple casts for each role, termed "responsibilities". If someone goes down, and you're responsible for that role, you go in. Unfortunately the smaller the company is, it creates what we lovingly call "the domino effect". As someone gets injured, you end up having to cut roles because you just don't have enough covers/responsible dancers.