One possible scenario could be that Jason is in a state of shock. He might be thinking about how this injury could affect his role in the mission. He could be worried about letting his friends down, especially since he's always seen as a strong leader. 'I can't believe this happened. How will I be able to fight like this?' he might be thinking as he looks at his injury.
Chainsaws in graphic novels can have symbolic meanings too. It could symbolize the breakdown of order or civilization. A character using a chainsaw might be on a path of chaos, cutting through the normalcy of the story's world. This can be used to explore themes of anarchy, survival, or the darker side of human nature.
No, Chainsaw Man is not a true story. It is a fictional manga and anime. It features a lot of supernatural elements like a character having chainsaws for arms which don't exist in real life.