If they are injured for the season, they can apply for another year of eligibility. Sometimes two.
They usually end up transferring institutions, but are allowed to continue their studies. A kid from my high school got a scholarship to play ball at Notre Dame, played a few years then ending up tearing his ACL after already getting an extra year of eligibility and was denied his request for his 6th year. In the meantime, he got his bachelors and MBA.
