Often, his friends like Sasuke, Sakura, and Kakashi might come to his aid. Sakura, being a medical ninja, would likely be the first to start treating his injuries. Sasuke could use his Sharingan to help in some way, perhaps to find the root of the injury or fend off any threats while Naruto is injured. Kakashi, with his experience, would also offer strategic advice on how to deal with the situation.
Sakura is a very likely character. As a skilled medical ninja, she has the knowledge and ability to treat Naruto's injuries. In many fanfictions, she is often the first to rush to his side and use her medical jutsu to stabilize him.
Another plotline could involve Naruto being injured as a result of a trap. Maybe he's exploring an ancient ruin in search of some power to help protect the village and he triggers a hidden mechanism. This could lead to him being severely wounded, and his friends would need to come rescue him while also trying to figure out how to disarm any other potential threats in the area.
In some fanfictions, Naruto's recovery could be a long and arduous journey. He might have to go through intense physical and mental training while recovering. The author could also introduce some kind of ancient and forgotten medical knowledge that is used to treat his severe injuries. His recovery might also be tied to some prophecy or hidden power within him that is awakened due to the near - death experience.
Sasuke is very likely to be involved. Given their complex relationship, Sasuke could be the one who finds Naruto severely injured and has to make a choice between helping him or leaving him. Another character could be Sakura. As a medic - ninja, she would be the one to try and heal Naruto's severe injuries. Kakashi too might be involved, either as a mentor figure who tries to figure out what happened to Naruto or as someone who has to step in to protect Naruto from further harm while he is injured.
One common plotline is a powerful enemy ambush. For example, Naruto might be on a solo mission and get caught off - guard by a new, extremely strong villain. This villain could use some unknown jutsu to severely wound Naruto. Another plotline could be Naruto sacrificing himself to protect the village. He might take on a massive attack meant for the Hidden Leaf, resulting in him getting severely injured but saving the village in the process. And sometimes, it could be due to a failed experiment. Maybe he was trying out a new and risky form of his chakra manipulation and it backfired, leaving him severely injured.
Another possibility is that Harry gets injured while exploring some dangerous part of the wizarding world, like an ancient and cursed ruin. He might be attacked by some dark creatures there, and his injuries are so severe that he has to rely on some rare magic or the help of very powerful wizards to heal.
When Harry is severely injured, he often becomes more vulnerable. This can make him more relatable. He might start to rely on his friends more, which deepens his relationships. For example, Hermione might use her vast knowledge to find a way to heal him, and this could strengthen their bond.
Often, a fanfiction might have Naruto severely wounded during a training accident that goes horribly wrong. He might be trying a new and extremely powerful jutsu that he's not fully mastered yet. This could result in him being in a critical condition. His friends and mentors then have to figure out how to heal him and deal with the consequences of his over - ambition in training.
Sakura is very likely to help. She's a skilled medical ninja and has always been there for Naruto. She would use her medical jutsu to try and heal his wounds. Kakashi too. He has a lot of experience and would be able to protect Naruto while he's injured. Also, Sasuke might come to Naruto's aid. Despite their past differences, they have a deep bond, and Sasuke would not let Naruto die if he could help it.