In a fanfiction, Mikey might get hurt during a mission to infiltrate a Kraang base. He could be detected by security systems and get zapped by some sort of electric defense mechanism. This would leave him weak and in pain. His brothers would need to carry him out of there and find a way to reverse the effects of the zap. They'd be desperate to get Mikey back to his normal, energetic self.
One possible scenario could be that Mikey gets hurt during a fight with the Foot Clan. Maybe he underestimated an opponent and got a nasty cut on his arm. His brothers would then rush to his side, with Donnie trying to patch him up as quickly as possible while Leo and Raph stand guard, ready to fend off any further attacks.
Leo would likely take charge immediately. He'd assess the situation and start giving orders on how to help Mikey. He'd be calm on the outside but worried on the inside.
I'm not sure specifically as I haven't read this particular fanfiction. But it might be about Mikey from the Teenage Mutant Ninja Turtles getting some sort of skin condition like shellacne. It could involve his friends helping him deal with it, or his own journey in handling this new and unexpected situation.
Leo would probably take charge immediately. He'd be focused on getting Mikey to safety and finding a way to treat his injuries. He might be blaming himself a bit for not being there to protect Mikey better.
In many 'TMNT Mikey hurt' fanfictions, a common plot element is Mikey's own attitude towards his injury. Sometimes he tries to be brave and not show how much pain he's in to not worry his brothers. Another element is the search for a cure or solution. If he's poisoned, they'll look for an antidote. If it's a physical wound, they'll try to find medical supplies or a way to heal it. And there's usually a lot of brotherly bonding as they take care of Mikey.
April and Casey might also be part of this fanfiction. April could use her reporting skills to try and figure out what's going on with Mikey's mutation, and Casey would be ready to fight any new threats that come along because of Mikey's changed state. Also, Splinter, as their sensei, would be guiding them all through this difficult time, providing wisdom and training to help Mikey control his new powers if he has any.
Well, his brothers would probably go on a wild rescue mission. They'd search every nook and cranny of the city. Maybe they'd get some help from April and Casey too.