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.
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.
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.