Well, Leo would be the calm and collected one showing concern. He'd organize everything from Mikey's care to the security of their lair. He might also give Mikey pep talks about getting better soon. Raph would show his concern through his actions. He'd be the first to volunteer to go out and get anything Mikey needs, whether it's medicine or his favorite food. And Donnie would be constantly hovering over Mikey with his medical equipment. He'd be making sure Mikey is taking the right medicine at the right time and monitoring his progress closely. All of them would also share memories with Mikey to keep his spirits high.
Leo would likely take charge. He'd be calm on the outside but worried inside. He'd make sure Donnie gets proper rest and tries to organize the others to help. Raph might be a bit gruff at first, but he'd actually be really concerned. He might offer to get supplies or guard Donnie while he sleeps. Mikey would be the most emotional. He'd probably be constantly by Donnie's side, trying to tell him jokes to make him feel better.
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.
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.
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.
He might gain new physical abilities, like enhanced strength or speed.
Splinter, as their sensei and father figure, would offer wisdom and comfort. He might have some traditional methods to help with Donnie's pain and healing. And he would also use this as a teaching moment for all of his sons about the importance of caution and looking out for each other.
It could be Shredder. He's always been after the turtles.
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.
In this fanfiction, Mikey might gain new and unexpected powers. Maybe he could develop the ability to control certain elements like water or fire, which would add a whole new dynamic to his character and the story.