He'd probably be a mix of angry and sad. Angry at whoever or whatever caused Naruto to be in this state, and sad because he cares about Naruto so much. He might clench his fists tightly, trying to control his emotions while he focuses on helping Naruto. He'd also feel a sense of guilt, wondering if he could have prevented this from happening.
One possibility is that Kakashi would immediately use his medical ninjutsu to treat Naruto's wounds if he has the skills. Then he'd try to calm Naruto down by talking softly to him, asking what happened. He might vow to protect Naruto better in the future and start investigating the cause of Naruto's harm.
One possible fanfiction could be about Kakashi getting injured during a mission. Maybe he's protecting his team and takes a powerful jutsu head - on. His students then have to figure out how to get him to safety and find a way to heal him. It could show their growth as they face this difficult situation.
Well, often in Hurt Kakashi fanfiction, there's the theme of his physical injuries and how he copes with them. Given his dangerous line of work as a ninja, he gets hurt a lot. Writers might explore how he deals with long - term or serious injuries that could potentially end his ninja career. Also, his relationships can be a source of pain. For instance, his complex relationship with Obito and how that affects his mental state when they are at odds.
Often, Gai is paired with Kakashi in Hurt Kakashi fanfiction. Gai, with his unwavering belief in friendship and his eternal rivalry with Kakashi, can be a great character to interact with a hurt Kakashi. He might try to pull Kakashi out of his slump through their friendly competition. Also, Rin is sometimes included. Since she has a significant past with Kakashi, her presence in the fanfiction can either be a source of comfort or more pain, depending on how the story is written.
Another situation is when Kakashi is on a solo mission. He might encounter a group of powerful opponents who manage to wound him. This forces him to rely on his wits and basic ninja skills to survive while waiting for backup. It can show his resourcefulness even when injured.
In a battle situation where they are on opposite sides, perhaps due to a misunderstanding or some kind of mind - control. Kakashi, with his superior skills, could accidentally harm Naruto while trying to subdue him. For example, if Naruto is being influenced by an enemy's genjutsu and attacks Kakashi, Kakashi might have to use a more forceful defensive move that ends up hurting Naruto.
In a fanfiction, Kakashi and Minato could be on a mission together in a dangerous, unknown territory. There could be traps everywhere. Minato, being the faster one, might rush ahead to scout but accidentally trigger a trap that causes some damage to him. Then, when Kakashi tries to help, he might also get injured by the after - effects of the trap or other hidden threats in the area.
Another reason could be related to Kakashi's past actions. If there are some secrets or things Kakashi did in the past that come to light and Naruto doesn't like them, he could snap at Kakashi. Naruto has a strong sense of justice and loyalty, so if he feels betrayed or misled by Kakashi, his reaction could be to snap.
A plot could be that an old enemy of Minato resurfaces. This enemy has a grudge against Minato and wants to take revenge. They set up an elaborate plan. Minato and Kakashi are lured into a trap. Minato gets hit by a poisoned weapon, and Kakashi gets injured while trying to fend off the enemy to buy time for Minato's treatment.