Spoilers ahead for Episode 10 of Chicago Fire Season 9, called "One Crazy Shift."
Casey took a pretty bad hit to the head earlier in Chicago Fire Season 9, and his bell was rung hard enough that his head trauma from all the way back in Season 2 is presenting very real problems again. After that initial injury, Casey learned that another head injury could "exacerbate trauma to a debilitating level," and depending on how he deals with it following "One Crazy Shift," this second hit to the head could have permanent consequences for him.
Unsurprisingly, Casey was still trying to keep his head trauma a secret in "One Crazy Shift," but he was feeling the effects enough that more than one person noticed that something was off, and Gallo even tried to check in with him about whether he was hiding something. There are four big ways that I can see Chicago Fire handling this story, and some more optimistic than the others. Let's start with the best-case scenario before diving into the darker possibilities!
Casey Finds A Fix
Casey has survived impossible odds more than once over the years, and it's hard to imagine a Chicago Fire without Jesse Spencer as Matt Casey. In fact, it's much easier to imagine that Fire can come up with some kind of fix to let Casey keep his job as CFD Captain, keep fighting fires, and not risk death if he gets another knock on his noggin. Whether this is via a treatment or a discovery of a misdiagnosis or Casey getting new safety gear, Fire could simply find a fix for him. Will Halstead's cameo from Chicago Med did see him suggesting a visit with a neurologist for Casey's "friend," after all!
Casey Stops Being A Firefighter
If Chicago Fire sticks with the prognosis of Casey's second head trauma, then this might be the end of Captain Casey of Truck 81 fighting fires in the field, and Boden already holds Firehouse 51's place as the CFD officer who doesn't generally run into burning buildings. There are plenty of CFD characters who don't actively fight fires, though, so it's not out of the realm of possibility that Casey embraces his own accountability about his injury and moves to a different (and safer) position at the CFD. It's also possible that instead of doing the mature thing, Casey will keep hiding the injury until it gets worse to the point of collapse, and perhaps loses his job because of it.
Casey Leaves Chicago
I've actually had a theory for a while that Chicago Fire is setting up Jesse Spencer's departure as Matt Casey, as the show has seemingly abandoned the Brettsey arc, which was arguably Casey's biggest running plot for the past couple seasons, and is revisiting the injury. Throw in the mentions of Dawson and reminders of the Dawsey relationship with Dawson conveniently off-screen, and I can see Casey realizing that he has to leave his job as a firefighter because of the injury and deciding to join Dawson rather than stick around Chicago. This theory is reliant on Casey doing some soul-searching, but even he might have to face the reality of his injury sooner rather than later if Fire isn't going to give him a magic fix.
Casey is doing something incredibly dangerous by continuing to act as a CFD captain in the field while suffering from a serious head injury, and it's entirely possible that doing so will lead to his death. Whether this is because he became too dizzy and out-of-it to do his job in the middle of a fire or if he collapses while fighting that fire, it's easy to imagine something going wrong for Casey yet again. I truly hope that Chicago Fire isn't going to kill Casey off after everything he's gone through over the years, and I'm going to knock on wood after even writing it, but it could happen.
Personally, I'm torn on which of these scenarios I prefer. I don't want to lose Casey from Chicago Fire, especially when the show has at least two more seasons left and I love seeing Casey and Severide bro-ing it up on a weekly basis, so I won't complain if there's a magic fix to his head. That said, I also love when long-running TV shows deliver game-changing twists to mix things up, and Fire actually delivering consequences on this could be great for the story.
If we are going to lose Casey, I 100% hope that he decides to go reunite with Dawson rather than stick around Chicago without the job that he loves. No matter where you stand on Dawsey, Dawson and Casey were Fire's central couple for the better part of the series until Monica Raymund left, and Fire revived the twinkly Dawsey song when Raymund returned as a guest star in Season 8 to show that the sparks still fly between them. I can see Dawson as a happy ending for Casey, and that's what I want for him more than anything else. And I think the stage is being set.