I don't think it is a good idea to have a new POV character for one scene.
You have control of history, don't injure your hero character, or don't injure her so badly, or go back in your story and figure out how to delay the battle until she can participate.
It sounds like you are a discovery writer, inventing the plot as you go. So am I, but sometimes I write myself into a dead-end, as you have done. The answer is to unwind back to a turning point and fix it.
Stephen King is a discovery writer, and he did the same thing while writing The Stand: Wrote himself into a corner; his story stalled. After thinking about it for a week and thinking he was going to lose the book, he scrapped a hundred pages of writing and rewound. He decided that, because he lets his characters do what he thinks is the most natural thing for each to do, they became complacent: So he went back to where he decided that first began (the stall), and he had the bad guys plant a bomb that killed half his heroic crew. He introduced a major plot twist.
In the final story, you can't tell that happened: It reads smooth. The bonb is a surprise but it makes sense: the bad guys wanted to hit them and did it, using a traitor and sabotage. The traitor was unhappy with the good guys; I don't know if he added that or just used what was there.
Just go back and fix it. Think of something else. The Hero has to be at the battle; whatever you wanted to accomplish with the injury, accomplish it some other way. Make it happen to someone else, maybe someone the hero loves (as a friend or romantic interest). Do like King, and come up with a plot twist, something unexpected (that still makes sense).