How about do something like keep 14 or 15 including the injured girl, and have two or three other girls that are "alternates" to be used if girls on the roster become injured or not eligible due to grades, or quit the team for some reason.
Might work, but might get messy. Kids sign up for high school softball to play and be part of the real team, not practice all the time with no uniform and no hope of seeing the field. So that's a hard sell, and then the player feels pressure to do it because it might affect whether she's on the team next year. And then let's say one of the varsity players that you thought was the 12th best player turns out to be the 18th best, and the 18th best who is an alternate suddenly looks like the 12th best. Awkward for coach and players.