One common theme could be emotional hurt. Leo might be betrayed by a friend or a loved one, which causes him great emotional pain. This could lead to him becoming withdrawn or having trust issues.
Raph usually gets really angry. He blames himself for not being there to protect Leo or he gets mad at the enemy who hurt Leo. He'll be the first to want revenge and will be very protective of Leo while he's injured.
Another theme is self - discovery. Through the pain, Leo might start to understand himself better. For example, he could realize his limits and also find new strengths within himself that he didn't know he had before.
Finding 'leo hurt fanfiction' can be a bit tricky as it depends on the fandom. If Leo is from a popular series, for example, a superhero or a fantasy series, there might be more available. You could also try fanfiction.net. However, not all works on there are of high quality. So, you need to read the summaries and reviews to find the good ones.
A lesser - known but very well - written 'leo valdez hurt fanfiction' is 'Hurt and Healing for Leo'. In this, Leo gets injured in a battle and the story is about his recovery process. It shows his vulnerability as well as his strength as he battles not only his physical wounds but also his self - doubt.
Raph would be the first to show his concern through anger. He'd be mad at himself for not protecting Leo better and also at the situation that led to Leo's injury. Mikey would be visibly distraught, constantly by Leo's side trying to cheer him up. Donnie would be all business, using his scientific knowledge to figure out the best way to treat Leo's wounds and help him recover.
One common scenario could be during a battle with a powerful enemy. For example, the Foot Clan might set up a complex trap and Leo, being the leader, might rush in to protect his brothers and get seriously injured. Another could be in a training accident. Maybe he's trying a new and difficult move and it goes wrong, resulting in an injury.
Maybe Leo has some sort of premonition or information that he believes is right, but it leads to Chris getting hurt. Like, Leo thinks that a certain magical training method is good for Chris, but it actually backfires and causes Chris pain. It could also be that there's a misunderstanding between them. Leo misinterprets Chris's actions as something malicious and takes actions against him that cause harm.